Superconducting device and superconducting cable

ABSTRACT

A superconducting device according to the present invention has an oxide superconducting wire. The sintering density of an oxide superconductor in the oxide superconducting wire is at least 93%, preferably at least 95%, and more preferably at least 99%. Thus, a superconducting device capable of suppressing ballooning also upon temperature increase without temperature control can be obtained.

TECHNICAL FIELD

The present invention relates to a superconducting device and asuperconducting cable, and more particularly, it relates to asuperconducting device and a superconducting cable capable ofsuppressing ballooning also upon temperature increase withouttemperature control.

BACKGROUND ART

When a superconducting device such as a superconducting cable is used,the superconducting device is dipped in a liquid refrigerant such asliquid nitrogen or liquid helium, for example, and held at a cryogenictemperature for cooling a superconductor filament in the superconductingdevice to below the critical temperature (T_(c)). On the other hand, thesuperconducting device is taken out from the liquid refrigerant at thetime of inspection or the like, for example, and a gas refrigerant orthe like of the room temperature is fed around the superconductingdevice for heating the device from the cryogenic temperature to the roomtemperature. When the device is heated to the room temperature after thesame is dipped in the liquid refrigerant, however, the following problemarises in the conventional superconducting device.

Small pinholes are generally present on the surface of an oxidesuperconducting wire constituting the superconducting device. When thisoxide superconducting wire is dipped in the refrigerant over a longperiod, the liquid refrigerant infiltrates into gaps of thesuperconductor filament in the oxide superconducting wire through thepinholes. When the temperature is increased to the normal temperaturefrom this state, the liquid refrigerant infiltrating into the oxidesuperconducting wire is vaporized and the vaporized gas is notdischarged if the rate of temperature increase is excessive. Thus, theinternal pressure of the oxide superconducting wire is increased toexpand the oxide superconducting wire (resulting in ballooning). Whenballooning takes place, the superconductor filament is disadvantageouslybroken to result in characteristic reduction such as reduction of thecritical current density.

In this relation, Japanese Patent Laying-Open No. 2002-260458 (PatentLiterature 1), for example, discloses a vaporization rate control methodof inhibiting a superconducting cable from ballooning. The vaporizationrate control method disclosed in the aforementioned gazette is a methodof controlling the vaporization rate of a refrigerant by setting therate of temperature increase for a superconductor of the superconductingcable to not more than 10 K/hour. More specifically, the vaporizationrate of the refrigerant is controlled by setting the rate of temperatureincrease for the superconductor of the superconducting cable to not morethan 10 K/hour through means for supplying the refrigerant flowing intothe superconducting cable at a temperature higher than that in ordinarycooling, means for supplying the refrigerant flowing into thesuperconducting cable at a flow rate smaller than that in ordinarycooling, means for introducing a temperature-increasing fluid of atemperature exceeding that of the refrigerant in ordinary cooling intothe refrigerant fed to the superconducting cable or means for supplyingthe refrigerant to the superconducting cable while gradually increasingthe pressure of the refrigerant from a state where the temperature ofthe refrigerant is not more than and close to the boiling point. Thus,the vaporization rate of the liquid refrigerant infiltrating into thesuperconducting wire is so relatively reduced that expansion of the wirecan be suppressed.

Patent Literature 1: Japanese Patent Laying-Open No. 2002-260458

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

In the method disclosed in the aforementioned gazette, however, the ratefor heating the superconductor from the cryogenic temperature to theroom temperature must be controlled, and temperature control intemperature increase is complicated. Further, the rate of temperatureincrease for the superconductor is at such a small level of not morethan 10 K/hour that a long time is required for increasing thetemperature.

The following method is also conceivable as a method capable ofsuppressing ballooning following temperature increase withouttemperature control. In other words, a method of suppressing ballooningby plating the periphery of a sheath of an oxide superconducting wireconstituting a superconducting device with a metal for blocking pinholesthereby inhibiting a liquid refrigerant from infiltrating into gaps of asuperconductor filament in the oxide superconducting wire is alsoconceivable.

According to this method, however, the weight of the superconductingdevice is increased by the plated metal, to increase the size of thesuperconducting device. Further, the number of manufacturing steps forthe superconducting device is also increased.

Accordingly, an object of the present invention is to provide asuperconducting device and a superconducting cable capable ofsuppressing ballooning also upon temperature increase withouttemperature control.

Means for Solving the Problems

The superconducting device according to the present invention has anoxide superconducting wire. The sintering density of an oxidesuperconductor in the oxide superconducting wire is at least 93%.

The superconducting cable according to the present invention has anoxide superconducting wire. The sintering density of an oxidesuperconductor in the oxide superconducting wire is at least 93%.

According to each of the inventive superconducting device and theinventive superconducting cable, the number of gaps in the oxidesuperconductor is so extremely small that a liquid refrigerant hardlyinfiltrates into the gaps of the oxide superconductor. When thetemperature is increased from a state dipped in the liquid refrigerantto the ordinary temperature without temperature control, therefore, thequantity of vaporized liquid refrigerant is extremely small.Consequently, the internal pressure of the oxide superconducting wire ishardly increased and ballooning can be suppressed.

Preferably in the superconducting device according to the presentinvention, the sintering density of the oxide superconductor in theoxide superconducting wire is at least 95%.

Preferably the superconducting cable according to the present invention,the sintering density of the oxide superconductor in the oxidesuperconducting wire is at least 95%.

Thus, the number of gaps in the oxide superconductor is so extremelyreduced that the liquid refrigerant more hardly infiltrates into thegaps of the oxide superconductor. When the temperature is increased froma state dipped in the liquid refrigerant to the ordinary temperaturewithout temperature control, therefore, ballooning can be moresuppressed.

Preferably in the superconducting device according to the presentinvention, the sintering density of the oxide superconductor in theoxide superconducting wire is at least 99%.

Preferably in the superconducting cable according to the presentinvention, the sintering density of the oxide superconductor in theoxide superconducting wire is at least 99%.

Thus, the number of gaps in the oxide superconductor is so extremelyreduced that the liquid refrigerant further hardly infiltrates into thegaps of the oxide superconductor. When the temperature is increased froma state dipped in the liquid refrigerant to the ordinary temperaturewithout temperature control, therefore, ballooning can be furthersuppressed.

The oxide superconducting wire having the oxide superconductorexhibiting the aforementioned sintering density can be manufactured bythe following manufacturing method:

A wire having a configuration obtained by covering raw material powderfor the oxide superconductor with a metal is prepared. The wire isheat-treated in a pressurized atmosphere. The total pressure of thepressurized atmosphere is at least 1 MPa and less than 50 MPa.

According to the inventive method of manufacturing an oxidesuperconducting wire, plastic flow and creep deformation ofsuperconducting crystals formed in the heat treatment result from thelarge external pressure of at least 1 MPa for the wire, whereby thenumber of gaps between the oxide superconducting crystals is reduced(sintering density of the oxide superconductor is improved). Further,gas in gaps of oxide superconducting crystal powder formed in the heattreatment or gas adhering to the oxide superconducting crystal powderformed in the heat treatment can be inhibited from expansion in the heattreatment due to a pressure from outside a metal tube, whereby the oxidesuperconducting wire is inhibited from blistering. Consequently, thecritical current density is improved.

In order to form a stable oxide superconducting phase, the partialoxygen pressure must be regularly controlled in a constant rangeregardless of the value of the total pressure in the pressurizedatmosphere. If the total pressure in the pressurized atmosphere exceeds50 MPa in this case, however, the partial oxygen pressure with respectto the total pressure is reduced. Thus, the value of the oxygenconcentration in the pressurized atmosphere is so extremely reduced thatthe same is strongly influenced by a measurement error or the like, andhence the partial oxygen pressure is disadvantageously hard to control.According to the inventive method of manufacturing an oxidesuperconducting wire, the heat treatment is performed in the pressurizedatmosphere of less than 50 MPa, whereby the partial oxygen pressure withrespect to the total pressure in the pressurized atmosphere is notexcessively reduced but the oxygen concentration in the pressurizedatmosphere is at a somewhat high level, whereby the partial oxygenpressure is easily controlled with no significant influence by themeasurement error or the like. An oxide superconducting wire having anoxide superconductor exhibiting a sintering density of at least about93% and not more than about 96% is obtained by heat-treating the wire inthe pressurized atmosphere having the total pressure of at least 1 MPaand less than 50 MPa.

Preferably in the aforementioned method of manufacturing an oxidesuperconducting wire, the heat treatment step is carried out through hotisostatic pressing (HIP).

Thus, the oxide superconducting wire is so isotropically pressurizedthat the same is homogeneously prevented from gaps and blisters.

Preferably in the aforementioned method of manufacturing an oxidesuperconducting wire, the oxide superconductor is aBi—Pb—Sr—Ca—Cu—O-based oxide superconductor including a Bi2223 phasecontaining bismuth, lead, strontium, calcium and copper at atomic ratiosof (bismuth and lead):strontium:calcium:copper expressed as 2:2:2:3 inapproximation.

Thus, gaps between crystals as well as blistering of the oxidesuperconducting wire are so suppressed that the critical current densitycan consequently be improved.

Preferably in the aforementioned method of manufacturing an oxidesuperconducting wire, the heat treatment step is carried out in anoxygen atmosphere, and the partial oxygen pressure is at least 0.003 MPaand not more than 0.02 MPa.

Thus, the partial oxygen pressure is so kept in the range of at least0.003 MPa and not more than 0.02 MPa that a stable oxide superconductingphase is formed and the critical current density can be improved. Ahetero phase is formed if the partial oxygen pressure exceeds 0.02 MPa,while the oxide superconducting phase is hard to form and the criticalcurrent density is reduced if the partial oxygen pressure is less than0.003 MPa.

Preferably in the aforementioned method of manufacturing an oxidesuperconducting wire, the partial oxygen pressure is controlled to beincreased following temperature rise in the pressurized atmosphere intemperature increase before the heat treatment in the heat treatmentstep.

The value of the partial oxygen pressure optimum for forming the oxidesuperconducting phase is increased following temperature rise. Thus, aproper partial oxygen pressure is attained also in the temperatureincrease before the heat treatment in the heat treatment step, whereby astable oxide superconducting phase is formed and the critical currentdensity can be improved.

Preferably in the aforementioned method of manufacturing an oxidesuperconducting wire, the total pressure in the pressurized atmosphereis controlled to be constant in the heat treatment.

In the heat treatment, the total pressure may exhibit a decreasingtendency due to consumption of oxygen gas resulting from oxidation of abearer supporting the wire in a pressurizing vessel, hunting of apressure regulator such as a dwelling valve during pressure control orpressure fluctuation in introduction of gas added for supplementingconsumed oxygen. If this results in abrupt decompression in the vessel,the internal pressure of the wire reaches a high level as compared withthe external pressure, to blister the wire. According to the preferredaspect of the present invention, however, the total pressure in the heattreatment is controlled to be constant, whereby the wire can beprevented from blistering resulting from abrupt decompression in theheat treatment.

Preferably in the aforementioned method of manufacturing an oxidesuperconducting wire, the heat treatment step is carried out in anoxygen atmosphere, and the partial oxygen pressure in the heat treatmentis controlled to be constant in a fluctuation range within 10%.

Thus, the partial oxygen pressure can be kept in the range optimum forforming an oxide superconducting phase regardless of temperaturefluctuation, whereby a stable oxide superconducting phase is formed andthe critical current density can be improved.

Preferably in the aforementioned method of manufacturing an oxidesuperconducting wire, gas is injected for supplementing pressurereduction resulting from temperature reduction in temperature reductionimmediately after the heat treatment.

In the temperature reduction immediately after the heat treatment,pressure reduction follows temperature change. If a heating vessel isabruptly decompressed at this time, the internal pressure of the wire isincreased as compared with the external wire, to blister the wire.According to the preferred aspect of the present invention, however, thegas is injected to supplement pressure reduction resulting fromtemperature reduction, whereby the wire can be prevented from blisteringresulting from abrupt decompression in the temperature reductionimmediately after the heat treatment.

Preferably in the aforementioned method of manufacturing an oxidesuperconducting wire, the decompression rate in the temperaturereduction immediately after the heat treatment is controlled to not morethan 0.05 MPa/min. if the metal covering the raw material powderincludes silver and the ratio (hereinafter referred to as a silverratio) of the area of a metal portion to the area of an oxidesuperconductor portion in a cross section of the wire after the heattreatment step is 1.5.

Thus, a more remarkable effect of preventing the wire from blisteringresulting from abrupt decompression can be attained when the silverratio is 1.5.

Preferably in the aforementioned method of manufacturing an oxidesuperconducting wire, the decompression rate for the total pressure inthe pressurized atmosphere is controlled to not more than 0.05 MPa/min.when the temperature in the atmosphere is at least 200° C. in the heattreatment step, if the metal covering the raw material powder includessilver and the silver ratio is 1.5.

If the heating vessel is abruptly decompressed when the temperature inthe atmosphere is at least 200° C., the internal pressure of the wire isincreased as compared with the external pressure, to blister the wire.Therefore, a more remarkable effect of inhibiting the wire fromblistering resulting from abrupt decompression in the heat treatmentstep (before the heat treatment, in the heat treatment and after theheat treatment) is attained when the silver ratio is 1.5.

Preferably in the aforementioned method of manufacturing an oxidesuperconducting wire, the decompression rate in temperature reductionimmediately after the heat treatment is controlled to not more than 0.03MPa/min., if the metal covering the raw material powder includes silverand the silver ratio is 3.0.

Thus, a more remarkable effect of preventing the wire from blisteringresulting from abrupt decompression can be attained when the silverratio is 3.0.

Preferably in the aforementioned method of manufacturing an oxidesuperconducting wire, the decompression rate for the total pressure inthe pressurized atmosphere is controlled to not more than 0.03 MPa/min.if the metal covering the raw material powder includes silver, thesilver ratio is 3.0 and the temperature in the atmosphere is at least200° C. in the heat treatment step.

If the heating vessel is abruptly decompressed when the temperature inthe atmosphere is at least 200° C., the internal pressure of the wire isincreased as compared with the external pressure, to blister the wire.Therefore, a more remarkable effect of inhibiting the wire fromblistering resulting from abrupt decompression in the heat treatmentstep (before the heat treatment, in the heat treatment and after theheat treatment) is attained when the silver ratio is 3.0.

Preferably in the aforementioned method of manufacturing an oxidesuperconducting wire, the decompression rate for the total pressure inthe pressurized atmosphere is controlled to not more than 0.05 MPa ifthe total pressure of the pressurized atmosphere is at least 1 MPa inthe heat treatment step.

If the heating vessel is abruptly decompressed when the total pressurein the atmosphere is at least 1 MPa, the internal pressure of the wireis increased as compared with the external pressure, to blister thewire. Therefore, a more remarkable effect of inhibiting the wire fromblistering resulting from abrupt decompression in the heat treatmentstep (before the heat treatment, in the heat treatment and after theheat treatment) is attained.

The aforementioned method of manufacturing an oxide superconducting wirepreferably further comprises a step of rolling the wire with a rollafter the step of preparing the wire and before the heat treatment step,and the skin thickness of the wire after the rolling step is at least 20μm.

Pinholes are mainly formed by holes externally penetrating toward anoxide superconductor filament when the surface of the wire is rougheneddue to friction between the wire and the roll. When the wire is rolledin such a state that the skin thickness of the oxide superconductingwire is at least 20 μm in every portion in the rolling step, however, noholes externally penetrate toward the oxide superconductor filament evenif the surface of the wire is roughened by rolling, to form no pinholes.Thus, formation of gaps and blisters is suppressed through theaforementioned heat treatment step, and the critical current density canbe improved. Throughout the specification, the term “pinhole” denotes ahole, externally penetrating toward the oxide superconducting wirefilament, having a diameter allowing passage of the liquid refrigerant.Further, the term “wire having pinholes” denotes a wire including atleast two pinholes in a length of 1000 m.

The aforementioned method of manufacturing an oxide superconducting wirepreferably further comprises a step of applying silver or silver alloyto the surface of the said wire after the step of preparing the wire andbefore the heat treatment step.

The silver ratio of an oxide superconducting wire is minimized in orderto increase the quantity of superconducting current feedable per unitarea. In a wire having a small silver ratio, however, the ratio of ametal portion is so small that the skin thickness cannot be increased.Particularly in a wire having a skin thickness of less than 20 μm aftera heat treatment step, pinholes are easily formed in treatment such asrolling before the heat treatment step. In the wire having pinholes,pressurizing gas infiltrates into the wire through the pinholes alsowhen the aforementioned step of heat-treating the wire in thepressurized atmosphere is carried out. Therefore, difference between theinternal and external pressures of the wire disappears to result in asmall effect of preventing reduction of the critical current density bysuppressing formation of gaps and blisters by pressurization. Thus,silver or a silver alloy is so applied to the surface of the wire afterthe step of preparing the wire and before the heat treatment step sothat pinholes are covered with the silver or silver alloy and disappearfrom the surface. Therefore, the heat treatment step is carried outafter removing pinholes from the wire, whereby no pressurizing gasinfiltrates into the wire through pinholes in the heat treatment step.Thus, formation of gaps and blisters is suppressed through theaforementioned step of heat-treating the wire in the pressurizedatmosphere, so that the critical current density can be improved.

The aforementioned method of manufacturing an oxide superconducting wirepreferably further comprises a step of rolling the wire with a rollafter the step of preparing the wire and before the heat treatment step,and the surface roughness Ry of a portion of the roll coming intocontact with the wire is not more than 320 nm.

Thus, friction between the wire and the roll is so reduced that thesurface of the wire is hardly roughened and a wire having no pinholes isobtained regardless of the skin thickness thereof. Therefore, nopressurizing gas infiltrates into the wire through pinholes in the heattreatment step. Thus, formation of gaps and blisters is suppressedthrough the aforementioned step of heat-treating the wire in thepressurized atmosphere regardless of the skin thickness of the wire, sothat the critical current density can be improved. The term “surfaceroughness Ry” denotes the maximum height defined in JIS (JapaneseIndustrial Standards).

Preferably in the aforementioned method of manufacturing an oxidesuperconducting wire, the pressure is controlled to be increasedstepwise following temperature rise in the atmosphere in temperatureincrease before the heat treatment in the heat treatment step.

In a wire having pinholes, pressurizing gas infiltrates into the wirethrough the pinholes also when the step of heat-treating the wire in apressurized atmosphere is carried out by an ordinary pressurizingmethod, and hence difference between the internal and external pressuresof the wire disappears to result in a small effect of preventingreduction of the critical current density resulting from formation ofgaps and blisters by pressurization. When the pressure is controlled tobe increased stepwise following temperature rise in the atmosphere,however, the external pressure is increased before the pressurizing gasinfiltrates into the wire through pinholes. Thus, difference takes placebetween the internal and external pressures of the wire, so thatformation of gaps and blisters is suppressed and the critical currentdensity can be improved whether or not the wire has pinholes before theheat treatment step.

Preferably in the aforementioned method of manufacturing an oxidesuperconducting wire, the total pressure of the atmosphere is controlledto be increased at a rate of at least 0.05 MPa/min. in temperatureincrease before the heat treatment in the heat treatment step.

The inventors have found that the speed of the pressurizing gasinfiltrating into the wire through pinholes in the step of heat-treatingthe wire is less than about 0.05 MPa/min. Therefore, the pressure in theatmosphere can be regularly kept higher than the internal pressure ofthe wire by controlling the total pressure of the atmosphere to becontinuously increased at a rate of at least 0.05 MPa/min. intemperature increase before the heat treatment. Thus, compressive forcecan be applied to the wire in temperature increase before the heattreatment whether or not the wire has pinholes in advance of the heattreatment step, whereby formation of gaps and blisters is suppressed.Consequently, reduction of the critical current density can beeffectively suppressed by the heat treatment in the pressurizedatmosphere of at least 1 MPa and less than 50 MPa.

Preferably in the aforementioned method of manufacturing an oxidesuperconducting wire, the total pressure in the atmosphere is controlledto be continuously increased in the heat treatment in the heat treatmentstep.

Thus, equalization of the internal pressure of the wire and the pressureof the atmosphere can be retarded in the heat treatment, so that thestate where the pressure in the atmosphere is higher than the internalpressure of the wire can be kept for a longer period. Therefore,formation of gaps and blisters is suppressed in the heat treatment, andreduction of the critical current density can be effectively suppressedthrough the heat treatment in the pressurized atmosphere of at least 1MPa and less than 50. MPa.

The aforementioned method of manufacturing an oxide superconducting wirepreferably further comprises a step of rolling the wire after the stepof preparing the wire and before the heat treatment step, and the draftof the wire in the rolling step is not more than 84%, more preferablynot more than 80%.

When the step of heat-treating the wire is carried out in thepressurized atmosphere of at least 1 MPa and less than 50 MPa, the oxidesuperconducting wire is compressed also in the heat treatment step. Alsowhen the step of rolling the wire is carried out with a draft of notmore than 84% lower than a conventional draft, therefore, the rawmaterial powder is compressed in the subsequent heat treatment step,whereby the density of the superconductor filaments can consequently beincreased. On the other hand, the step of rolling the wire is carriedout with the draft of not more than 84% lower than the conventionaldraft so that gaps are hardly formed in the raw material powder, wherebyformation of gaps extending perpendicularly to the longitudinaldirection of the oxide superconducting wire can be suppressed. Thus, thecritical current density of the oxide superconducting wire can beimproved. Further, the step of rolling the wire is so carried out withthe draft of not more than 80% that no gaps are formed in the rawmaterial powder, whereby formation of gaps extending perpendicularly tothe longitudinal direction of the oxide superconducting wire can be moresuppressed.

Throughout this specification, the draft (%) is defined as follows:

[Num 1]Draft (%)=(1−thickness of wire after rolling/thickness of wire beforerolling)×100

Preferably in the aforementioned method of manufacturing an oxidesuperconducting wire, a plurality of times of heat treatment areperformed on the wire, and at least one of the plurality of times ofheat treatment is performed in a pressurized atmosphere having a totalpressure of at least 1 MPa and less than 50 MPa.

Thus, gaps between oxide superconducting crystals formed in the heattreatment and blistering of the oxide superconducting wire can besuppressed.

An oxide superconducting wire having an oxide superconductor exhibitinga higher sintering density can be manufactured by the followingmanufacturing method. Further, the oxide superconducting wire can bereformed to an oxide superconducting wire having an oxide superconductorexhibiting a higher sintering density by the following reforming method:

The method of manufacturing an oxide superconducting wire according tothe present invention comprises a step of preparing a wire having aconfiguration obtained by covering raw material powder for an oxidesuperconductor with a metal and a heat treatment step of heat-treatingthe wire in a pressurized atmosphere having a total pressure of at least1 MPa and less than 50 MPa in heat treatment. In temperature increasebefore the heat treatment in the heat treatment step, pressurization isstarted from a temperature at which 0.2% yield strength of the metalemployed for covering is smaller than the total pressure in the heattreatment.

The method of reforming an oxide superconducting wire according to thepresent invention comprises a heat treatment step of heat-treating anoxide superconducting wire having a configuration obtained by coveringan oxide superconductor with a metal in a pressurized atmosphere havinga total pressure of at least 1 MPa and less than 50 MPa in heattreatment. In temperature increase before the heat treatment in the heattreatment step, pressurization is started from a temperature at which0.2% yield strength of the metal employed for covering is smaller thanthe total pressure in the heat treatment.

According to the inventive manufacturing method or the inventivereforming method for the oxide superconducting wire, a pressure isapplied to the wire in the state where the 0.2% yield strength of themetal employed for covering is smaller than the total pressure of thepressurized atmosphere in the heat treatment. Thus, a metal portionreceiving compressive force resulting from pressurization is easilycompressed due to an effect similar to that of hot working. Therefore,the wire is compressed before pressurizing gas infiltrates into the wirethrough pinholes, whereby formation of gaps and blisters can besufficiently suppressed by pressurization. Consequently, the sinteringdensity of the oxide superconductor can be improved, so that thecritical current density of the oxide superconducting wire can beimproved.

Another method of manufacturing an oxide superconducting wire accordingto the present invention comprises a step of preparing a wire having aconfiguration obtained by covering raw material powder for an oxidesuperconductor with a metal including silver and a heat treatment stepof heat-treating the wire in a pressurized atmosphere having a totalpressure of at least 1 MPa and less than 50 MPa in heat treatment. Intemperature increase before the heat treatment in the heat treatmentstep, pressurization is started after the temperature of the atmosphereexceeds 400° C.

Another method of reforming an oxide superconducting wire according tothe present invention comprises a heat treatment step of heat-treatingan oxide superconducting wire having a configuration obtained bycovering an oxide superconductor with a metal including silver in apressurized atmosphere having a total pressure of at least 1 MPa andless than 50 MPa in heat treatment. In temperature increase before theheat treatment in the heat treatment step, pressurization is startedafter the temperature of the atmosphere exceeds 400° C.

According to the inventive manufacturing method or the inventivereforming method for the oxide superconducting wire, a pressure isapplied to the wire in a state where 0.2% yield strength of the metalincluding silver is reduced to a degree identical to the total pressureof the pressurized atmosphere in the heat treatment. Thus, a metalportion receiving compressive force resulting from pressurization iseasily compressed due to an effect similar to that of hot working.Therefore, the wire is compressed before pressurizing gas infiltratesinto the wire through pinholes, whereby formation of gaps and blisterscan be sufficiently suppressed by pressurization. Consequently, thesintering density of the oxide superconductor can be improved, so thatthe critical current density of the oxide superconducting wire can beimproved. An oxide superconducting wire having an oxide superconductorexhibiting a sintering density of at least 95% is obtained by theaforementioned manufacturing method or the aforementioned reformingmethod, regardless of pinholes.

Preferably in each of the aforementioned manufacturing method and theaforementioned reforming method, pressurization is started after thetemperature of the atmosphere exceeds 600° C. in temperature increasebefore the heat treatment in the heat treatment step.

Thus, a pressure is applied to the wire in a state where 0.2% yieldstrength of the metal including silver is reduced to about half thetotal pressure of the pressurized atmosphere in the heat treatment.Thus, a metal portion receiving compressive force resulting frompressurization is more easily compressed. Consequently, the sinteringdensity of the oxide superconductor can be more improved, so that thecritical current density of the oxide superconducting wire can be moreimproved. An oxide superconducting wire having an oxide superconductorexhibiting a sintering density of at least 97% is obtained by theaforementioned manufacturing method or the aforementioned reformingmethod, regardless of pinholes.

Preferably in each of the aforementioned manufacturing method and theaforementioned reforming method, the pressing speed is at least 0.05MPa/min.

The inventors have found that the speed of the pressurizing gasinfiltrating into the wire through pinholes in the heat treatment stepis less than about 0.05 MPa/min. Therefore, the pressure in theatmosphere can be regularly kept higher than the internal pressure ofthe wire by controlling the total pressure of the atmosphere to becontinuously increased at a rate of at least 0.05 MPa/min. intemperature increase before the heat treatment. Thus, compressive forcecan be applied to the wire in temperature increase before the heattreatment whether or not the wire has pinholes in advance of the heattreatment step, whereby formation of gaps and blisters is suppressed.Consequently, the sintering density of the oxide superconductor can beeffectively improved by the heat treatment in the pressurized atmosphereof at least 1 MPa and less than 50 MPa, and the critical current densityof the oxide superconducting wire can be effectively improved.

Preferably in each of the aforementioned manufacturing method and theaforementioned reforming method, the rate of pressurization is at least0.1 MPa/min.

Thus, the pressure in the atmosphere can be kept further higher than theinternal pressure of the wire. Therefore, compressive force can befurther largely applied to the wire in temperature increase before theheat treatment whether or not the wire has pinholes in advance of theheat treatment step, whereby formation of gaps and blisters issuppressed. Consequently, the sintering density of the oxidesuperconductor can be more effectively improved by the heat treatment inthe pressurized atmosphere of at least 1 MPa and less than 50 MPa, andthe critical current density of the oxide superconducting wire can bemore effectively improved.

When the pressing speed is set to at least 0.15. MPa/min. in both casesof starting pressurization after the temperature of the atmospherereaches 400° C. and 600° C. respectively, an oxide superconducting wirehaving an oxide superconductor exhibiting a sintering density of atleast 99% is obtained regardless of pinholes.

Preferably in the aforementioned manufacturing method, the raw materialpowder for the oxide superconductor includes a Bi2223 phase, and theoxide superconducting wire is annealed in an atmosphere containingoxygen at a temperature of at least 100° C. and not more than 600° C. inthe heat treatment step.

Thus, the critical current density J_(c) at a low temperature of about20 K is improved in the overall wire.

Effect of the Invention

According to the inventive superconducting device, the number of gaps inthe oxide superconductor is so extremely small that a liquid refrigeranthardly infiltrates into the gaps of the oxide superconductor. When thetemperature is increased from a state dipped in the liquid refrigerantto the ordinary temperature without temperature control, therefore, thequantity of vaporized liquid refrigerant is extremely small.Consequently, the internal pressure of the oxide superconducting wire ishardly increased and ballooning can be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1A] A sectional view of a superconducting cable according to afirst embodiment of the present invention.

[FIG. 1B] An enlarged view of a cable core in FIG. 1A.

[FIG. 2] A partially fragmented perspective view conceptually showingthe structure of an oxide superconducting wire.

[FIG. 3] A diagram showing steps of manufacturing the oxidesuperconducting wire.

[FIG. 4] A schematic sectional view of a hot isostatic pressing (HIP)apparatus.

[FIGS. 5(a) to 5(d)] Conceptual views showing the behavior of gapsbetween oxide superconducting crystals stepwise.

[FIG. 6] A diagram showing the relation between the total pressure P(MPa) of a pressurized atmosphere and the number of blisters in a wire(number/10 m).

[FIG. 7] A diagram showing total pressures and partial oxygen pressuresas to mixed gas prepared at a ratio of about 80% of nitrogen and about20% of oxygen.

[FIG. 8] A diagram showing the relation between total pressures andoxygen concentrations in a case of setting a partial oxygen pressureconstant.

[FIG. 9A] A diagram showing the relation between time and thetemperature of a wire in a case of performing decompression rate controlimmediately after heat treatment.

[FIG. 9B] A diagram showing the relation between time and a totalpressure in a vessel in the case of performing decompression ratecontrol immediately after heat treatment.

[FIG. 10A] A graph showing the thicknesses of oxide superconductingwires having no pinholes before and after heat treatment in apressurized atmosphere.

[FIG. 10B] A graph showing the thicknesses of oxide superconductingwires having pinholes before and after heat treatment in a pressurizedatmosphere.

[FIG. 11] A partially fragmented perspective view conceptually showingthe structure of an oxide superconducting wire having pinholes.

[FIG. 12] A schematic sectional view showing a rolling method in asecond embodiment.

[FIG. 13] A diagram showing other steps of manufacturing an oxidesuperconducting wire.

[FIG. 14] A partially fragmented perspective view conceptually showingthe structure of an oxide superconducting wire after a step of platingthe wire with silver or a silver alloy.

[FIG. 15] A diagram showing the relation between temperatures andpressures in heat treatment and time in a fourth technique in the secondembodiment.

[FIG. 16A] A diagram showing the relation between the temperature in aheat treatment step and time in a case where a silver ratio is 1.5 inthe second embodiment of the present invention.

[FIG. 16B] A diagram showing the relation between the pressure in theheat treatment step and time in the case where the silver ratio is 1.5in the second embodiment of the present invention.

[FIG. 16C] A diagram showing the relation between the oxygenconcentration in the heat treatment step and time in the case where thesilver ratio is 1.5 in the second embodiment of the present invention.

[FIG. 16D] A diagram showing the relation between the partial oxygenpressure in the heat treatment step and time in the case where thesilver ratio is 1.5 in the second embodiment of the present invention.

[FIG. 17] A diagram showing the relation between temperatures andpressures in a heat treatment step and time in a fifth technique in thesecond embodiment of the present invention.

[FIG. 18] A diagram showing the optimum combination of a temperature anda partial oxygen pressure in heat treatment.

[FIG. 19] A partially fragmented perspective view conceptually showingthe structure of an oxide superconducting wire having gaps remainingtherein.

[FIG. 20] A diagram schematically showing the relation between draftsand critical current densities in primary rolling in oxidesuperconducting wires.

[FIG. 21] A diagram showing exemplary relation between temperatures,total pressures and partial oxygen pressures in temperature increasebefore heat treatment and in heat treatment and time in a sixthtechnique in a sixth embodiment of the present invention.

[FIG. 22] A diagram showing the relation between pressing speeds andsintering densities with reference to various temperatures for startingpressurization.

[FIG. 23] A diagram showing temperature dependency of 0.2% yieldstrength of silver.

[FIG. 24] A diagram showing the relation between sintering densities ofoxide superconductors and critical current values of oxidesuperconducting wires.

[FIG. 25] A diagram showing exemplary relation between temperatures,total pressures and partial oxygen pressures and time in a case ofperforming annealing after heat treatment in a seventh embodiment of thepresent invention.

[FIG. 26] A diagram showing critical current values I_(c) at respectivetemperatures (K) of oxide superconducting wires before annealing andafter annealing performed at a temperature of 500° C.

DESCRIPTION OF REFERENCE NUMERALS

1, 1 a, 1 b oxide superconducting wire, 2 oxide superconductor filament,3 sheath portion, 4 gas inlet, 5 upper lid, 6 cylindrical vesselcylinder, 7 thermal barrier, 8 treated product, 9 heater, 10 bearer, 11lower lid, 12 superconducting crystal, 13 hot isostatic press, 14pinhole, 15 roll, 15 a surface of roll, 16 silver or silver alloy, 20gap, 30 superconducting cable, 31 cable core, 32 former, 34 insulatingpaper, 35 kraft paper, 37 refrigerant passage, 38 adiabatic tube, 39anticorrosive layer.

BEST MODES FOR CARRYING OUT THE INVENTION

Embodiments of the present invention are now described with reference tothe drawings.

FIRST EMBODIMENT

FIG. 1A is a sectional view of a superconducting cable according to afirst embodiment of the present invention, and FIG. 1B is an enlargedview of a cable core in FIG. 1A.

Referring to FIGS. 1A and 1B, a superconducting cable 30 comprises cablecores 31, an adiabatic tube 38 and an anticorrosive layer 39. Eachsingle-filamentary or multifilamentary stranded cable core 31 isinserted in a refrigerant passage 37 formed inside the adiabatic tube 38and the anticorrosive layer 39. A refrigerant is circulated along theouter periphery of the cable core 31 in the refrigerant passage 37. Thecable core 31 is constituted of a former (a plurality of copper strands)32, a plurality of oxide superconducting wires 1 a, kraft paper 35,another plurality of oxide superconducting wires 1 b and insulatingpaper 34 successively from the inside. The tapelike oxidesuperconducting wires 1 a and 1 b are spirally wound on the outerperiphery of the former 32 composed of a plurality of copper strandshaving an outer diameter of 20 mm, for example. The plurality of oxidesuperconducting wires 1 a and the plurality of oxide superconductingwires 1 b forming a laminated structure are insulated from each otherthrough the kraft paper 35. In the lower layer of the plurality of oxidesuperconducting wires 1 a, 13 oxide superconducting wires 1 a arearranged at a pitch of 200 mm, for example. In the upper layer of theplurality of oxide superconducting wires 1 b, 14 oxide superconductingwires 1 a are arranged at a pitch of 200 mm, for example. Each of theoxide superconducting wires 1 a and 1 b has a rectangular section of0.21 mm by 4.1 mm, for example. The oxide superconducting wires 1 b areexternally covered with the insulating paper 34 formed by polypropylenelaminated paper (PPLP(R)), for example.

The structure of each oxide superconducting wire constituting thesuperconducting cable is now described.

FIG. 2 is a partially fragmented perspective view conceptually showingthe structure of an oxide superconducting wire.

A multifilamentary oxide superconducting wire, for example, is describedwith reference to FIG. 2. The oxide superconducting wire 1 has aplurality of longitudinally extending oxide superconductor filaments 2and a sheath portion 3 covering the same. The material for each of theplurality of oxide superconductor filaments 2 preferably has aBi—Pb—Sr—Ca—Cu—O-based composition, for example, and a materialincluding a Bi2223 phase having atomic ratios of (bismuth andlead):strontium:calcium:copper expressed substantially as 2:2:2:3 inapproximation is particularly optimum. The material for the sheathportion 3 is composed of silver, for example.

While the multifilamentary wire has been described in the above, anoxide superconducting wire having a single-filamentary structure formedby a single oxide superconductor filament 2 covered with a sheathportion 3 may alternatively be employed.

A method of manufacturing the aforementioned oxide superconducting wireis now described.

FIG. 3 is a diagram showing steps of manufacturing the oxidesuperconducting wire.

Referring to FIG. 3, raw material powder for an oxide superconductor isfirst charged into a metal tube (step S1). This raw material powder forthe oxide superconductor is composed of a material including a Bi2223phase, for example.

The metal tube is preferably prepared from silver or a silver alloyhaving high thermal conductivity. Thus, heat generated when thesuperconductor partially causes quenching can be quickly removed fromthe metal tube.

Then, the metal wire charged with the raw material powder is worked intoa wire of a desired diameter by wire drawing (step S2). Thus, a wirehaving a configuration obtained by covering the raw material powder forthe oxide superconducting wire with a metal is obtained. In order tomanufacture a multifilamentary wire, a plurality of drawn wires areinserted into a metal tube, to be further subjected to wire drawing.Primary rolling is performed on this wire (step S3), and first heattreatment is thereafter performed (step S4). An oxide superconductingphase is formed from the raw material powder through these operations.Secondary rolling is performed on the heat-treated wire (step S5). Thus,voids resulting from the first heat treatment are removed. Second heattreatment is performed on the secondarily rolled wire (step S6).Sintering of the oxide superconducting phase progresses and the oxidesuperconducting phase is singularized at the same time through thesecond heat treatment.

The oxide superconducting wire shown in FIG. 2, for example, can bemanufactured by the aforementioned method.

According to this embodiment, at least either the first heat treatment(step S4) or the second heat treatment (step S6) is performed in apressurized atmosphere to which a pressure of at least 1 MPa and lessthan 50 MPa is applied as a total pressure.

The heat treatment in this pressurized atmosphere is performed by hotisostatic pressing (HIP), for example. This hot isostatic pressing isnow described.

FIG. 4 is a schematic sectional view of a hot isostatic pressing (HIP)apparatus.

Referring to FIG. 4, an apparatus 13 for carrying out hot isostaticpressing is constituted of a pressure vessel cylinder 6, an upper lid 5and a lower lid 11 closing both ends of the pressure vessel cylinder 6,a gas inlet 4 provided on the upper lid 5 for introducing gas into thepressure vessel cylinder 6, a heater 9 heating a treated product 8, athermal barrier 7 and a bearer 10 supporting the treated product 8.

According to this embodiment, the bearer 10 supports a wire prepared bycharging a metal tube with raw material powder and thereafterwire-drawing and rolling the same in the pressure vessel cylinder 6 asthe treated product 8. In this state, prescribed gas is introduced intothe pressure vessel cylinder 6 from the gas inlet 4, thereby forming apressurized atmosphere of at least 1 MPa and less than 50 MPa in thepressure vessel cylinder 6 so that the wire 8 is heated with the heater9 to a prescribed temperature under this pressurized atmosphere. Thisheat treatment is preferably performed in an oxygen atmosphere, and thepartial oxygen pressure is preferably at least 0.003 MPa and not morethan 0.02 MPa. Thus, heat treatment by hot isostatic pressing isperformed on the wire 8.

According to this embodiment, the heat treatment is performed in thepressurized atmosphere of at least 1 MPa and less than 50 MPa asdescribed above, for mainly attaining the following three effects:

First, the number of gaps formed between oxide superconducting crystalsin the heat treatment can be reduced.

The inventor has found that the number of gaps between oxidesuperconducting crystals mainly formed in heat treatment can beremarkably reduced by performing the heat treatment in a pressurizedatmosphere of at least 1 MPa as compared with a case of less than 1 MPa.

FIGS. 5(a) to 5(d) are conceptual diagrams showing behavior of gapsbetween oxide superconducting crystals stepwise.

Referring to FIGS. 5(a) to 5(d), the contact area between the oxidesuperconducting crystals formed in the heat treatment is increased byplastic flow when the heat treatment is performed in a pressurizedatmosphere, to reduce the number of gaps of several μm to several 10 μmpresent between the superconducting crystals (FIG. 5(a)→FIG. 5(b)). Whenthis state is held, creep deformation is caused as shown in FIG. 5(c) tocontract the gaps present on a junction interface while a contaminatedportion such as an oxide film is partially broken/decomposed to causediffusion of atoms and progress sintering. Finally, most of the gapsbetween the superconducting crystals disappear as shown in FIG. 5(d), toform a stable junction interface.

To feed a current to a superconducting wire means to feed the currentbetween superconducting crystals constituting the superconducting wire.In general, a junction between the superconducting crystals exhibiting aweak superconducting state (superconducting crystals are stronger insuperconductivity than the junction between the crystals) limits thequantity of current feedable while maintaining the superconducting state(causing no electrical resistance) in a refrigerant (liquid nitrogen orliquid helium, for example) for using the superconducting wire or inrefrigerator cooling. Gaps inevitably remain in the junction between thesuperconducting crystals in ordinary atmospheric pressure baking.Therefore, the number of the gaps between the superconducting crystalscan be reduced (sintering density of the superconductor can be improved)by the heat treatment in the pressurized atmosphere, whereby theperformance of the superconducting wire is improved and reduction of thecritical current density can be prevented.

More specifically, the sintering density of an oxide superconductorheat-treated in the atmospheric pressure is 80 to 90% as to an oxidesuperconducting wire including a Bi2223 phase, while the sinteringdensity of an oxide superconductor filament prepared by themanufacturing method according to the present invention by setting thetotal pressure of the pressurized atmosphere to 10 MPa was at least 93%and reduction of the number of gaps formed between oxide superconductorcrystals was recognized.

An oxide superconducting wire obtained by the aforementionedmanufacturing method is so applied to a superconducting device such as asuperconducting cable that a liquid refrigerant hardly infiltrates intogaps of the oxide superconductor. Also when the temperature is increasedwithout temperature control when the superconducting device is heatedfrom a state dipped in the liquid refrigerant to the ordinarytemperature, therefore, the liquid refrigerant is hardly vaporized.Consequently, the internal pressure of the oxide superconducting wire ishardly increased and the superconducting device such as asuperconducting cable can be inhibited from ballooning.

Second, the oxide superconducting wire can be prevented from blisteringresulting from the heat treatment.

The inventor has investigated the number of blisters formed in aheat-treated wire when the total pressure was varied in heat treatmentof an oxide superconducting wire in a pressurized atmosphere. FIG. 6 isa diagram showing the relation between the total pressure P (MPa) of thepressurized atmosphere and the number of blisters (number/10 m) in thewire.

Referring to FIG. 6, it is understood that the number of blisters in theoxide superconducting wire is remarkably reduced when the total pressureof the pressurized atmosphere exceeds 0.5 MPa and the blisters in theoxide superconducting wire substantially disappear when the totalpressure exceeds 1 MPa. These results have conceivably been obtained forthe following reason:

Powder of an oxide superconductor in a metal tube generally has afilling factor of about 80% of theoretical density before sintering, andhence gas is present in gaps of the powder. The gas in the gaps of thepowder is cubically expanded when heated to a high temperature in heattreatment, to blister the wire. According to this embodiment, however,the heat treatment is performed in the pressurized atmosphere of atleast 10 MPa, whereby the external pressure of the metal tube can berendered larger than the internal pressure of the metal tube. Thus, thewire is conceivably prevented from blistering resulting from the gas inthe gaps of the powder.

The inventor has further studied the cause for blistering of the wire,to also recognize that adsorbates such as carbon (C), water (H₂O) andoxygen (O₂) adhering to the raw material powder for the oxidesuperconductor are vaporized during sintering to expand the volume inthe metal tube and blister the wire with the gas. However, thisblistering of the wire resulting from vaporization of the adsorbates ofthe powder is also conceivably preventable by performing the heattreatment in the pressurized atmosphere of at least 1 MPa so that theexternal pressure can be rendered larger than the internal pressure ofthe metal tube.

Thus, not only blistering resulting from gas present in the gaps of theraw material powder for the oxide superconductor but also blisteringresulting from vaporization of adsorbates adhering to the surfaces ofgrains thereof can conceivably be substantially eliminated by settingthe total pressure of the pressurized atmosphere to at least 1 MPa.Blistering of the oxide superconducting wire causes reduction of thecritical current density, and hence reduction of the critical currentdensity can be prevented by preventing the wire from blistering.

Third, control of the partial oxygen pressure in the heat treatment canbe simplified.

The inventor has found that a 2223 phase of a Bi-based oxidesuperconductor is stably formed when the partial oxygen pressure iscontrolled to at least 0.003 MPa and not more than 0.02 MPa, regardlessof the total pressure. In other words, a hetero phase such as Ca₂PbO₄ isformed if the partial oxygen pressure exceeds 0.02 MPa while a Bi2223phase is hard to form if the partial oxygen pressure is less than 0.003MPa, to reduce the critical current density.

FIG. 7 is a diagram showing total pressures and partial oxygen pressuresas to mixed gas prepared at a ratio of about 80% of nitrogen and about20% of oxygen. FIG. 8 is a diagram showing the relation between totalpressures and oxygen concentrations in a case of setting the partialoxygen pressure constant.

Referring to FIG. 7, the partial oxygen pressure is equivalent to thelevel of 0.2 atm. (0.02 MPa) shown by a dotted line when the totalatmosphere of the pressurized atmosphere is at the atmospheric pressureof 1 atm. (0.1 MPa), for example, whereby a Bi2223 phase is stablyformed without partial oxygen pressure control. As the total pressure ofthe pressurized atmosphere is increased to 2 atm. (0.2 MPa), 3 atm. (0.3MPa) . . . , however, the partial oxygen pressure is also increased toexceed the level of 0.2 atm. (0.02 MPa) shown by the dotted line:Consequently, the Bi2223 phase is not stably formed. Therefore, thepartial oxygen pressure must be controlled to at least 0.003 MPa and notmore than 0.02 MPa by changing the mixing ratio of oxygen gas in themixed gas, as shown in FIG. 8. The dotted line in FIG. 8 shows the levelof 0.2 atm. (0.02 MPa), similarly to the dotted line in FIG. 7.

In practice, the partial oxygen pressure is controlled by monitoring thetotal pressure and the oxygen concentration. In other words, the partialoxygen pressure is calculated by multiplying the value of the totalpressure by the oxygen concentration. Thus, if the total pressure is 50MPa, for example, the oxygen concentration is 0.01% when the heattreatment is performed with a partial oxygen pressure of 0.005 MPa.Therefore, the injected mixed gas must be controlled by measuring theoxygen concentration of 0.01%. However, measurement of the oxygenconcentration of 0.01% leads to a large measurement error, and hence itis difficult to correctly control the partial oxygen pressure in atreatment chamber by controlling the oxygen gas in the injected mixedgas. According to this embodiment, the oxygen concentration can be keptat a level less influenced by a measurement error by setting the totalpressure in the pressurized atmosphere to less than 50 MPa, whereby thepartial oxygen pressure can be easily controlled.

When the heat treatment is performed in a pressurized atmosphere of atleast 1 MPa, the decompression rate is preferably so controlled that noabrupt decompression takes place in the pressurized atmosphere in theheat treatment and after the heat treatment.

When the heat treatment is performed in the pressurized atmosphere of atleast 1 MPa, external gas conceivably infiltrates into the wire throughpinholes of the wire to equalize the internal and external pressures ofthe wire with each other. The inventor has found that gas emission fromthe inside cannot follow reduction of the external pressure and theinternal pressure exceeds the external pressure to form blisters whenthe external pressure is reduced due to abrupt decompression in such ahigh-pressure atmosphere.

In order to prevent such blistering, therefore, mixed gas of inert gassuch as Ar (argon) or N₂ (nitrogen) and O₂ gas is preferably injectedinto the vessel in the heat treatment so that the total pressure isconstant. In temperature reduction immediately after the heat treatment,further, the mixed gas of inert gas and oxygen gas is injected into thevessel to supplement reduction of the pressure resulting fromtemperature reduction. Formation of blisters resulting from abruptdecompression can be prevented by controlling these decompression ratesin the heat treatment and the temperature reduction immediately afterthe heat treatment.

FIG. 9A is a diagram showing the relation between time and thetemperature of a wire subjected to decompression rate controlimmediately after heat treatment. FIG. 9B is a diagram showing therelation between time and the total pressure in a vessel subjected todecompression rate control immediately after heat treatment.

Referring to FIGS. 9A and 9B, the total pressure is controlled to beconstant as shown in FIG. 9B in the heat treatment (temperature of about800° C.) shown in FIG. 9A. In other words, oxygen gas in the vessel isconsumed in the heat treatment due to oxidation of a bearer supportingthe wire in a heating vessel or the like, and hence the pressure in thevessel is reduced. In order to prevent this, the mixed gas is injectedinto the vessel for keeping the pressure constant. In temperaturereduction (temperature range of about 800° C. to about 300° C.)immediately after the heat treatment shown in FIG. 9A, mixed gas isinjected into the vessel for supplementing reduction of the pressureresulting from temperature reduction as shown in FIG. 9B, forcontrolling the decompression rate. In other words, the pressure of thegas also starts to abruptly lower on the basis of a state equation ofgas due to reduction of the temperature in the temperature reduction,and hence decompression must be slowed down by injecting the mixed gas.In the range of not more than 300° C., the pressure in the wire isalready sufficiently low since the temperature is low as compared withthe case of about 800° C. to about 300° C. Therefore, the wire isconceivably not blistered also when the decompression rate is notcontrolled.

The inventor has further found that the range of the decompression ratenecessary for preventing the oxide superconducting wire from blisteringvaries with the ratio (silver ratio) of the area of a metal portion tothe area of an oxide superconductor portion in a cross section of thewire after the heat treatment. In other words, the decompression rate inthe temperature reduction (temperature range of 800° C. to 300° C.)immediately after the heat treatment is not more than 0.05 MPa/min. ifthe silver ratio is 1.5, while the decompression rate in the temperaturereduction (temperature range of 800° C. to 300° C.) immediately afterthe heat treatment is not more than 0.03 MPa/min. if the silver ratio is3.0.

While the method of manufacturing an oxide superconducting wire having aBi2223 phase by hot isostatic pressing is described with reference tothis embodiment, the present invention can also be carried out by apressurizing method other than hot isostatic pressing so far as the sameis a method of performing heat treatment in a pressurized atmosphere ofat least 1 MPa and less than 50 MPa. Further, the present invention isalso applicable to a method of manufacturing an oxide superconductingwire having another composition such as a yttrium-based compositionother than the bismuth-based composition.

SECOND EMBODIMENT

FIG. 10A is a graph showing thicknesses of oxide superconducting wireshaving no pinholes before and after heat treatment in a pressurizedatmosphere. FIG. 10B is a graph showing thicknesses of oxidesuperconducting wires having pinholes. Conditions for the heat treatmentin FIGS. 10A and 10B are a total pressure of 20 MPa, a partial oxygenpressure of 0.008 MPa, a temperature of 825° C. in an atmosphere and aheat treatment time of 50 hours.

Referring to FIG. 10A, the thickness of each oxide superconducting wirehaving no pinholes is reduced by about 0.006 mm to 0.01 mm after theheat treatment. This is because the number gaps between oxidesuperconducting crystals is reduced and the oxide superconducting wireis inhibited from blistering due to the heat treatment in thepressurized atmosphere of the total pressure of 20 MPa. Referring toFIG. 10B, on the other hand, the thickness is reduced only by about0.002 mm to 0.005 mm after the heat treatment in each oxidesuperconducting wire having pinholes, and reduction of the number ofgaps between oxide superconducting crystals and suppression ofblistering of the oxide superconducting wire are insufficientlyattained. Further, the thickness of a portion (portion A) of the wirehaving included pinholes is increased after the heat treatment ascompared with that before the heat treatment.

Thus, it has been recognized that formation of gaps and blisters can beeffectively suppressed by heat treatment in the pressure range (at least1 MPa and less than 50 MPa) of the first embodiment when no pinholes arepresent, while formation of gaps and blisters cannot be sufficientlysuppressed simply by the heat treatment in the pressure range of thefirst embodiment when pinholes are present.

In the heat treatment in the pressurized atmosphere according to thepresent invention, plastic flow and creep deformation of superconductingcrystals formed in the heat treatment result from the large pressure ofat least 1 MPa outside the wire, whereby gaps between oxidesuperconducting crystals formed in the heat treatment are suppressed.Further, gas in gaps of oxide superconducting crystal powder formed inthe heat treatment and gas adhering to the oxide superconducting crystalpowder formed in the heat treatment can be inhibited from expansion inthe heat treatment due to the pressure from outside a metal tube,whereby the oxide superconducting wire is inhibited from blistering.Consequently, reduction of the critical current density resulting fromgaps or blisters is prevented.

In a wire having pinholes, however, pressurizing gas infiltrates intothe wire through the pinholes despite the heat treatment in theaforementioned pressurized atmosphere, and hence no difference remainsbetween the internal and external pressures of the wire and formation ofgaps and blisters is not sufficiently suppressed by pressurization.Consequently, the effect of preventing reduction of the critical currentdensity is reduced.

Therefore, the inventors have made a deep study, to find techniquescapable of sufficiently suppressing formation of gaps and blisters byforming a wire having no pinholes before heat treatment.

According to a first technique, the skin thickness of an oxidesuperconducting wire is set to at least 20 μm after the rolling (step S3or S5) and before the heat treatment (step S4 or step S6) in FIG. 3.

According to a second technique, the surface roughness Ry of portions ofrolls, employed for the rolling (step S3 or S5) shown in FIG. 3, cominginto contact with the wire is set to not more than 320 nm.

According to a third technique, the oxide superconducting wire is platedwith silver or a silver alloy after the rolling (step S3 or S5) andbefore the heat treatment (step S4 or S6) in FIG. 3.

These techniques are now specifically described.

The inventors have found that no pinholes are formed in rolling (step S3or S5) when the skin thickness W of the oxide superconducting wire isset to at least 20 μm in every portion after the rolling (step S3 or S5)and before the heat treatment (step S4 or S6) in FIG. 3 as the firsttechnique.

FIG. 11 is a partially fragmented perspective view conceptually showingthe structure of an oxide superconducting wire having pinholes.

The skin thickness W denotes the distance W between oxide superconductorfilaments 2 arranged on an outer peripheral portion in a section of awire 1 and the outer surface of the wire 1 after rolling, as shown inFIG. 11. No pinholes 14 are formed when the skin thickness W is set toat least 20 μm, conceivably for the following reason:

Pinholes 14 are mainly formed by holes externally penetrating toward theoxide superconductor filaments 2 when the surface of the wire 1 isroughened due to friction between the wire 1 and pressure rolls. Whenthe oxide superconducting wire 1 is so rolled that the skin thickness Wis at least 20 μm in every portion after the rolling, however, no holesexternally penetrate toward the oxide superconductor filaments 2,whereby no pinholes 14 are conceivably formed. The structure shown inFIG. 11 is substantially identical to that shown in FIG. 2 except theaforementioned point, and hence identical members are denoted by thesame reference numerals, and redundant description is not repeated.

The inventors have found that a wire having no pinholes 14 is obtainedbefore heat treatment if the aforementioned second and third techniquesare employed also when the skin thickness W of the rolled oxidesuperconducting wire is less than 20 μm, and formation of gaps andblisters is consequently suppressed by heat treatment in a pressurizedatmosphere while reduction of the critical current density iseffectively prevented.

FIG. 12 is a schematic sectional view showing a rolling method accordingto the second embodiment.

Referring to FIG. 12, rolling is a working method of passing a plate- orbar-type material through a plurality of (generally two) rotating rolls15 for reducing the thickness or the sectional area thereof whileforming the section into a target shape. In the rolling, an oxidesuperconducting wire 1 is drawn into a clearance between the pluralityof rolls 15 due to frictional force from the rolls 15, and deformed bycompressive force from surfaces 15 a of the rolls 15.

In the second technique, the rolls 15 having surface roughness Ry of notmore than 320 nm on the surfaces 15 a coming into contact with the wire1 are employed in at least either the primary rolling (step S3) or thesecondary rolling (step S5) shown in FIG. 3.

In other words, friction between the wire 1 and the surfaces 15 a of therolls 15 is so reduced that the surface of the wire 1 is hardlyroughened and the wire 1 having no pinholes is obtained regardless ofthe skin thickness of the wire 1 if the surface roughness Ry of thesurfaces 15 a of the rolls 15 employed in rolling is not more than 320nm. Therefore, pressurizing gas does not infiltrate into the wire 1through pinholes in the heat treatment step. Thus, formation of gaps andblisters is suppressed through the aforementioned step of performingheat treatment in the pressurized atmosphere regardless of the skinthickness W of the wire 1, and reduction of the critical current densityis effectively prevented.

FIG. 13 is a diagram showing other steps of manufacturing an oxidesuperconducting wire.

In the third technique, a (step S11 or S12) of plating the surface of awire with silver or a silver alloy is carried out after rolling (step S3or S5) and before heat treatment (step S4 or S6), as shown in FIG. 13.This method is substantially identical to the method of FIG. 3 exceptthat the plating (step S11 or S12) is added, and hence correspondingsteps are denoted by corresponding reference numerals, and redundantdescription is not repeated.

FIG. 14 is a partially fragmented perspective view conceptually showingthe structure of an oxide superconducting wire after the step of platingthe wire with silver or a silver alloy.

Referring to FIG. 14, the outer periphery of a sheath portion 3 isplated with silver or a silver alloy 16, whereby externally openingpinholes 14 are blocked with the silver or silver alloy 16. Theremaining structure is substantially identical to the structure shown inFIG. 2, and hence identical members are denoted by the same referencenumerals, and redundant description is not repeated.

In general, the silver ratio of the oxide superconducting wire 1 isminimized in order to increase the quantity of a superconducting currentfeedable per unit area. In the wire 1 having a small silver ratio,however, the ratio of a metal portion is so small that the skinthickness W cannot be increased. Therefore, the skin thickness of thewire 1 having a small silver ratio is less than 20 μm, and the pinholes14 are easily formed in treatment (e.g. rolling) before the heattreatment step. In the wire 1 having the pinholes 14, formation of gapsand blisters is not sufficiently suppressed by pressurization, ashereinabove described. Consequently, the effect of preventing reductionof the critical current density is reduced. When the surface of the wire1 is plated with the silver or silver alloy 16 before the heat treatmentstep, the pinholes 14 are blocked with the silver or silver alloy 16, todisappear from the surface. Therefore, the heat treatment step iscarried out after the pinholes 14 disappear from the wire 1, whereby nopressurizing gas infiltrates into the wire 1 through the pinholes 14 inthe heat treatment step. Thus, formation of gaps and blisters issuppressed through the aforementioned step of performing heat treatmentin the pressurized atmosphere regardless of the value of the skinthickness W of the wire 1 and the value of the surface roughness Ry ofthe rolls 15 employed for rolling, and reduction of the critical currentdensity is effectively prevented.

The inventors have found that formation of gaps and blisters issuppressed (sintering density is improved) also in the wire 1 having thepinholes 14 and reduction of the critical current density is effectivelyprevented when a fourth technique or a fifth technique described belowis employed. In the fourth technique, the pressure is controlled toincrease stepwise following temperature rise in temperature increasebefore the heat treatment in at least either the first heat treatment(step S4) or the second heat treatment (step S6) shown in FIG. 3. In thefifth technique, the total pressure of the atmosphere is controlled toincrease at a rate of at least 0.05 MPa/min. in the temperature increasebefore the heat treatment in at least either the first heat treatment(step S4) or the second heat treatment (step S6) shown in FIG. 3. In theheat treatment, the total pressure of the atmosphere is controlled tocontinuously increase. In temperature reduction immediately after theheat treatment, further, control is made to supplement reduction of thepressure (to add a pressure) resulting from the temperature reduction.First, the fourth technique is described.

FIG. 15 is a diagram showing the relation between temperatures andpressures in heat treatment and time in the fourth technique of thesecond embodiment.

Referring to FIG. 15, the heat treatment is performed under conditionsof a heat treatment temperature of 800° C. and a pressure of 20 MPa. Atthis time, the pressure is controlled to increase stepwise followingtemperature rise. In other words, the pressure is controlled to repeat aprocess of increasing the pressure after keeping a prescribed pressurefor a constant time and keeping the increased pressure for a constanttime again in pressure increase. More specifically, the pressure is keptat about 7 MPa, about 10 MPa, about 12.5 MPa, about 15 MPa and about 17MPa for a constant time in the pressure increase process. The timing forincreasing the pressure after keeping the same for a constant time isdecided on the basis of a measured value of the temperature in theatmosphere. In other words, the pressure is so controlled as to increasethe pressure to about 7 MPa at the room temperature, increase thepressure to about 10 MPa when the temperature reaches about 400° C.,increase the pressure to about 12.5 MPa when the temperature reaches500° C., increase the pressure to about 15 MPa when the temperaturereaches 600° C. and increase the pressure to about 17 MPa when thetemperature reaches 700° C. In order to form a stable oxidesuperconducting phase, the partial oxygen pressure is controlled to beregularly in the range of 0.003 to 0.008 MPa.

In a wire having pinholes, pressurizing gas infiltrates into the wirethrough the pinholes when a step of performing heat treatment in apressurized atmosphere is carried out by a general pressurizing method,and hence no difference remains between the internal and externalpressures of the wire and an effect of preventing reduction of thecritical current density resulting from gaps and blisters bypressurization is small. When the pressure is controlled to increasestepwise following temperature rise in the fourth technique, however,the external pressure increases before the pressurizing gas infiltratesinto the wire through the pinholes. Thus, difference is caused betweenthe internal and external pressures of the wire, so that formation ofgaps and blisters is suppressed (sintering density is improved) andreduction of the critical current density is effectively preventedwhether or not the wire has pinholes before the heat treatment step.

Further, formation of gaps and blisters in the wire can be moreeffectively suppressed by combining the following technique with theaforementioned first to fourth techniques. This technique is nowdescribed.

In this technique, the decompression rate for the total pressure in thepressurized atmosphere is controlled to less than a constant rate in atleast either the first heat treatment (step S4) or the second heattreatment (step S6) shown in FIG. 3 if the temperature of the atmosphereis at least 200° C. in the heat treatment step.

FIG. 16A is a diagram showing the relation between the temperature inthe heat treatment step and time in a case where the silver ratio in thesecond embodiment of the present invention is 1.5. FIG. 16B is a diagramshowing the relation between the pressure in the heat treatment step andtime in the case where the silver ratio in the second embodiment of thepresent invention is 1.5. FIG. 16C is a diagram showing the relationbetween the oxygen concentration in the heat treatment step and time inthe case where the silver ratio in the second embodiment of the presentinvention is 1.5. FIG. 16D is a diagram showing the relation between thepartial oxygen pressure in the heat treatment step and time in the casewhere the silver ratio in the second embodiment of the present inventionis 1.5.

Referring to FIGS. 16A to 16D, the pressure is controlled to increasestepwise following temperature rise in the atmosphere in the temperatureincrease before the heat treatment, similarly to the aforementionedfourth technique. While the pressure appears not kept at a prescribedlevel for a constant time in FIG. 16B, the scale of the elapsed time inFIG. 16B is so larger than the scale in FIG. 15 that a pressure holdingpart seems to be omitted, and the pressure is kept at a prescribed levelfor a constant time similarly to the case of FIG. 15 in practice. Thetemperature and the pressure are set to 815° C. and 20 MPa respectivelythrough this temperature increase step, and heat treatment of 50 hoursis performed in this state. In temperature increase before the heattreatment and in the heat treatment, the decompression rate for thetotal pressure in the pressurized atmosphere is controlled to not morethan 0.05 MPa/min. if the temperature of the atmosphere is at least 200°C. After the heat treatment, the temperature is reduced at a rate of 50°C./h. Also after the heat treatment, the decompression rate for thetotal pressure in the pressurized atmosphere is controlled to not morethan 0.05 MPa/min. if the temperature of the atmosphere is at least 200°C. If the temperature reduction rate after the heat treatment is 50°C./h., a natural decompression rate following temperature reduction isregularly not more than 0.05 MPa/min., and hence the decompression ratemay not be controlled. Further, the oxygen concentration is kept at0.04% before the heat treatment, in the heat treatment and after theheat treatment. Thus, the partial oxygen pressure is regularly in therange of 0.003 to 0.008 MPa, and a stable oxide superconducting phasecan be formed.

If the heating vessel is abruptly decompressed when the temperature inthe atmosphere is at least 200° C., the internal pressure of the wire isincreased as compared with the external pressure, to blister the wire.When the decompression rate for the total pressure in the pressurizedatmosphere is controlled to less than a constant level, therefore, aneffect of inhibiting the wire from blistering resulting from abruptdecompression during the heat treatment (before the heat treatment, inthe heat treatment and after the heat treatment) is more remarkablyattained.

As to a wire having a silver ratio of 3.0, the decompression rate iscontrolled to not more than 0.03 MPa/min. when the temperature of theatmosphere is at least 200° C.

The fifth technique is now described. In the fifth technique, the totalpressure of the atmosphere is controlled to continuously increase at arate of at least 0.05 MPa/min. in temperature rise before the heattreatment in at least either the first heat treatment (step S4) or thesecond heat treatment (step S6) shown in FIG. 3. In the heat treatment,the total pressure of the atmosphere is controlled to continuouslyincrease. In temperature reduction immediately after the heat treatment,further, control is made to supplement reduction of the pressure (to adda pressure) resulting from the temperature reduction.

FIG. 17 is a diagram showing the relation between temperatures andpressures in the heat treatment step and time in the fifth technique ofthe second embodiment of the present invention.

Referring to FIG. 17, the pressure is slowly increased according to astate equation of gas in the temperature increase before the heattreatment if the temperature of the atmosphere is not more than 700° C.,for example. When the temperature of the atmosphere exceeds 700° C., thepressure of the atmosphere is increased to about 10 MPa. At this time,the pressure of the atmosphere is increased at a stroke at a pressingspeed of at least 0.05 MPa/min.

The inventors have found that the speed of the pressurizing gasinfiltrating into a wire through pinholes is less than about 0.05MPa/min. when an oxide superconducting wire having pinholes isheat-treated in a pressurized atmosphere. Therefore, the pressure of theatmosphere can be continuously kept higher than the internal pressure ofthe wire in temperature increase before the heat treatment bycontrolling the total pressure of the atmosphere to continuouslyincrease at a rate of at least 0.05 MPa/min. in the temperature increasebefore the heat treatment.

Thereafter the temperature is kept at 830° C., for example, in the heattreatment. On the other hand, the pressure of the atmosphere iscontinuously increased. While the pressing speed is preferably maximizedin the heat treatment, the total pressure exceeds 50 MPa if the pressingspeed is too high and hence the pressure must be continuously increasedat such a proper pressing speed that the total pressure does not exceed50 MPa in the heat treatment. Referring to FIG. 17, the pressure isincreased to about 30 MPa. Therefore, the time equalizing the internalpressure of the wire and the pressure in the atmosphere with each othercan be retarded from a time t₁ to a time t₂ as compared with a case ofkeeping the pressure constant in the heat treatment. Thus, the statewhere the pressure in the atmosphere is higher than the internalpressure of the wire can be kept for a longer time in the heattreatment.

Thereafter in the temperature reduction immediately after the heattreatment, the pressure also starts to lower along with reduction of thetemperature in the atmosphere according to the state equation of gas. Atthis time, the pressure is controlled to supplement reduction of thepressure resulting from the temperature reduction (to add a pressure).In order to form a stable oxide superconducting phase, the partialoxygen pressure is controlled to be regularly in the range of 0.003 to0.008 MPa.

According to the fifth technique, the pressure in the atmosphere exceedsthe internal pressure of the wire in the temperature increase before theheat treatment, whereby compressive force can be applied to the wire.Further, the state where the pressure in the atmosphere is higher thanthe internal pressure of the wire can be kept for a longer time in theheat treatment. Consequently, formation of gaps and blisters issuppressed in the temperature increase before the heat treatment and inthe heat treatment, and reduction of the critical current density can beeffectively suppressed through the heat treatment in the pressurizedatmosphere of at least 1 MPa and less than 50 MPa.

While the case of carrying out the step of plating the wire with thesilver or silver alloy has been described with reference to thisembodiment, the present invention can also be carried out through asputtering step, for example, so far as the silver or silver alloy isapplied to the wire in this step. In addition, while FIGS. 15 and 16A to16D show specific control conditions for temperatures, pressures, oxygenconcentrations and partial oxygen pressures, the present invention isnot restricted to these conditions but the pressure may be controlled toincrease stepwise following temperature increase, and the decompressionrate for the total pressure in the pressurized atmosphere may becontrolled to not more than 0.05 MPa/min. when the temperature in theatmosphere is at least 200° C.

Formation of pinholes can be prevented, or formation of gaps andblisters in the wire can be effectively suppressed upon formation ofpinholes, by combining the first to fifth techniques of this embodimentwith the heat treatment conditions of the first embodiment.

Formation of gaps and blisters of the wire can be more effectivelysuppressed by properly combining the first to fifth techniques of thisembodiment.

While the fifth technique of this embodiment has been described withreference to the case of making control to supplement reduction of thepressure (to add a pressure) resulting from the temperature reduction inthe temperature reduction immediately after the heat treatment, further,the present invention is not restricted to this case but the pressure inthe atmosphere may be controlled to continuously increase at least inthe heat treatment.

THIRD EMBODIMENT

In order to improve the critical current density of the oxidesuperconducting wire, the inventors have made a deep study as to anoptimum partial oxygen pressure in temperature increase before heattreatment and in the heat treatment. Thus, a result shown in FIG. 18 hasbeen obtained.

FIG. 18 is a diagram showing the optimum combination of a temperatureand a partial oxygen pressure in the heat treatment.

Referring to FIG. 18, it is understood that a stable oxidesuperconducting phase is formed and the critical current density isimproved in the temperature range of at least 815° C. and not more than825° C. when the partial oxygen pressure is 0.007 MPa, for example.While this is not shown in the figure, a stable oxide superconductingphase is formed and the critical current density is improved in thetemperature range of at least 750° C. and not more than 800° C.,preferably in the temperature range of at least 770° C. and not morethan 800° C. when the partial oxygen pressure is 0.003 MPa. When thepartial oxygen pressure is 0.02 MPa, a stable oxide superconductingphase is formed and the critical current density is improved in thetemperature range of at least 820° C. and not more than 850° C.,preferably in the temperature range of at least 830° C. and not morethan 845° C. Further, it has also been recognized that the partialoxygen pressure must be controlled in the range of at least 0.00005 MPaand not more than 0.02 MPa when the temperature is not more than 650° C.

From the aforementioned relation between the temperature and the partialoxygen pressure, the value of the partial oxygen pressure optimum forforming an oxide superconducting phase is increased followingtemperature rise. Therefore, the partial oxygen pressure can be set inthe range optimum for forming an oxide superconducting phase bycontrolling the partial oxygen pressure to increase followingtemperature rise in the atmosphere. Thus, a stable oxide superconductingphase is formed and the critical current density can be improved.

When the wire is held at a constant temperature in the heat treatment,fluctuation (error) of several ° C. is frequently caused in thetemperature. Considering the relation between this temperaturefluctuation and the optimum range of the partial oxygen pressure, theoptimum partial oxygen pressure is at least 0.006 MPa and not more than0.01 MPa when the wire is held at 822.5° C., for example, while theoptimum partial oxygen pressure is at least 0.07 MPa and not more than0.011 MPa when the temperature fluctuates to 825° C. When thetemperature fluctuates to 820° C., the optimum partial oxygen pressureis at least 0.005 MPa and not more than 0.009 MPa. Therefore, it followsthat the partial oxygen pressure may be controlled to be constant in thefluctuation range (slanted portion in FIG. 18) of at least 0.007 MPa andnot more than 0.009 MPa when the wire is held at 822.5° C., in order toregularly attain the optimum partial oxygen pressure despite suchtemperature fluctuation.

This fluctuation range of the partial oxygen pressure is about 10% ofthe value of the partial oxygen pressure. Therefore, the partial oxygenpressure in the heat treatment is so controlled to be constant in thefluctuation range within 10% that the partial oxygen pressure can be setin the optimum partial oxygen pressure range regardless of temperaturefluctuation, whereby a stable oxide superconducting phase is formed andthe critical current density can be improved.

While the exemplary numerical range of the optimum partial oxygenpressure in the temperature increase before the heat treatment and inthe heat treatment has been described with reference to this embodiment,the present invention is not restricted to the case of controlling thepartial oxygen pressure in this numerical range but the partial oxygenpressure may be controlled to increase following temperature rise in theatmosphere.

FOURTH EMBODIMENT

In order to further improve the critical current density of the oxidesuperconducting wire, the inventors have controlled the decompressionrate for a total pressure in heat treatment to 0.05 MPa/min., and made adeep study as to the relation between the value of the total pressureand blistering of the wire.

Raw material powder having composition ratios ofBi:Pb:Sr:Ca:Cu=1.82:0.33:1.92:2.01:3.02 was prepared. This raw materialpowder was heat-treated at 750° C. for 10 hours, and thereafterheat-treated at 800° C. for 8 hours. Thereafter powder obtained bypulverization was heat-treated at 850° C. for 4 hours, and thereafterpulverized again. Powder obtained by pulverization was heat-treatedunder decompression, and thereafter charged into a metal tube of silverhaving an outer diameter of 36 mm and an inner diameter of 31 mm. Then,the metal tube charged with the powder was subjected to wire drawing.Further, 61 drawn wires were bundled up and engaged in a metal tubehaving an outer diameter of 36 mm and an inner diameter of 31 mm. Then,wire drawing and primary rolling were performed for obtaining atape-like superconducting wire, having a Bi2223 phase, of 0.25 mm inthickness and 3.6 mm in width. Then, first heat treatment was performedon this wire. The first heat treatment was performed in the atmosphereat a heat treatment temperature of 842° C. for a heat treatment time of50 hours. Then, second heat treatment was performed after performingsecondary rolling. The second heat treatment was performed by settingthe partial oxygen pressure to 0.008 MPa, the heat treatment temperatureto 825° C. and the heat treatment time to 50 hours, controlling thedecompression rate for the total pressure in the heat treatment to notmore than 0.05 MPa/min. and varying the total pressure as shown inTable 1. After the second heat treatment, presence/absence of blistersin the wire was investigated. Table 1 shows the total pressures andpresence/absence of blisters in the wire. TABLE 1 Total Pressure (MPa)Expansion of Wire 0.1 no 0.2 no 0.3 no 0.4 no 0.5 no 0.8 no 1.0 yes 2.0yes 3.0 yes 5.0 yes 10.0 yes 20.0 yes 30.0 yes

From the results shown in Table 1, the wire is blistered when the totalpressure is at least 1 MPa. Thus, the decompression rate in thepressurized atmosphere must be controlled to not more than 0.05 MPa/min.when the total pressure is at least 1 MPa, in order to suppressblistering of the wire.

Then, the heat treatment temperature for the second heat treatment wasset to 500° C., for similarly investigating presence/absence of blistersof the wire. Table 2 shows the total pressures and presence/absence ofblisters of the wire. TABLE 2 Total Pressure (MPa) Expansion of Wire 0.1no 0.2 no 0.3 no 0.4 no 0.5 no 0.8 no 1.0 yes 2.0 yes 3.0 yes 5.0 yes10.0 yes 20.0 yes 30.0 yes

From the results shown in Table 2, the wire is blistered when the totalpressure is at least 1 MPa, also when the heat treatment temperature is500° C. Thus, the decompression rate in the pressurized atmosphere mustbe controlled to not more than 0.05 MPa/min. when the total pressure isat least 1 MPa also when the heat treatment temperature is 500° C., inorder to suppress blistering of the wire.

FIFTH EMBODIMENT

FIG. 19 is a partially fragmented perspective view conceptually showingthe structure of an oxide superconducting wire having gaps remainingtherein.

Referring to FIG. 19, gaps elongated in the longitudinal direction(transverse direction in FIG. 19) substantially disappear while gaps 20extending perpendicularly to the longitudinal direction slightly remainin a superconductor filament 2 of an oxide superconducting wire 1 afterheat treatment in a pressurized atmosphere having a total pressure of atleast 1 MPa and less than 50 MPa. FIG. 19 shows a single-filamentaryoxide superconducting wire having a single superconductor filament.

The inventors have found that the number of the gaps 20 extendingperpendicularly to the longitudinal direction of the oxidesuperconducting wire 1 is hard to reduce by the heat treatment in thepressurized atmosphere. This is conceivably for the following reason: Inthe pressurized atmosphere, a pressure is equivalently applied to allsurfaces of the oxide superconducting wire. Oxide superconductingcrystals cause creep deformation due to this pressure, to contract gapspresent on a junction interface between the crystals. Thus, the numberof gaps formed between the oxide superconducting crystals is reduced.However, the oxide superconducting wire 1 has a shape elongated in thelongitudinal direction, and hence force is hardly transmitted in thelongitudinal direction and the wire 1 is hardly compressed in thelongitudinal direction. Consequently, the number of the gaps 20extending perpendicularly to the longitudinal direction of the oxidesuperconducting wire 1 is hardly reduced by the heat treatment in thepressurized atmosphere.

The gaps 20 extending perpendicularly to the longitudinal direction ofthe oxide superconducting wire 1, blocking a current in thesuperconductor filament, are one of factors reducing the criticalcurrent density of the oxide superconducting wire 1. When formation ofthe gaps 20 is suppressed, therefore, the critical current density ofthe oxide superconducting wire 1 can be further improved.

Accordingly, the inventors have found that formation of gaps extendingperpendicularly to the longitudinal direction of the oxidesuperconducting wire can be suppressed before the heat treatment and thecritical current density of the oxide superconducting wire canconsequently be improved by setting the draft of the oxidesuperconducting wire to not more than 84%, preferably not more than 80%,in the primary rolling (step S5) in FIG. 3. The reason for this is nowdescribed.

The primary rolling is a step carried out for increasing the density ofthe raw material powder charged into the metal tube. As the draft of theoxide superconducting wire is increased (working ratio is increased) inthe primary rolling, the density of the raw material powder charged intothe metal tube is increased. When the density of the raw material powderis increased, the density of superconducting crystals formed by thesubsequent heat treatment (step S4 and step S5) is increased to improvethe critical current density of the oxide superconducting wire.

When the draft of the oxide superconducting wire is increased in theprimary rolling, however, the following three phenomena resulting fromthe increased working ratio may be recognized: First, gaps (cracks) areformed in the raw material powder. Second, sausaging is easily caused torender the shape of a filament in the oxide superconducting wirenonuniform in the longitudinal direction. Third, a portion of thesuperconductor filament having a locally increased sectional area due tothe sausaging easily comes into another superconductor filament to causebridging. All of these phenomena may serve as factors reducing thecritical current density of the oxide superconducting wire.

Therefore, the primary rolling must be performed with a draft increasingthe density of the raw material powder without forming gaps or the likein the raw material powder. In the conventional primary rolling, theoxide superconducting wire has been rolled with a draft of about 86 to90%.

If the heat treatment is performed in the pressurized atmosphere of atleast 1 MPa and less than 50 MPa, however, the effect of compressing theoxide superconducting wire is attained also in the heat treatment. Alsowhen the primary rolling is performed with a draft of not more than 84%,therefore, the raw material powder is compressed through the subsequentheat treatment in the pressurized atmosphere, whereby the density of thesuperconductor filament forming the oxide superconducting wire canconsequently be increased. On the other hand, the primary rolling isperformed with the draft of not more than 84% so that gaps are hardlyformed in the raw material powder, whereby formation of gaps extendingperpendicularly to the longitudinal direction of the oxidesuperconducting wire can be suppressed. In addition, the primary rollingis so performed with the draft of not more than 80% that absolutely nogaps are formed in the raw material powder. The critical current densityof the oxide superconducting wire can be improved for the above reasons.

FIG. 20 is a diagram schematically showing the relation between draftsand critical current densities in primary rolling in oxidesuperconducting wires.

Referring to FIG. 20, the critical current density of the oxidesuperconducting wire is maximized when heat treatment is performed inthe atmosphere and the primary rolling is performed with a draft ofabout 86%. When the heat treatment is performed in the pressurizedatmosphere of the present invention, on the other hand, the criticalcurrent density is maximized when the primary rolling is performed witha draft of about 82%. Thus, it is understood that optimum the draft forthe primary rolling for improving the critical current density of theoxide superconducting wire shifts to a lower side when the heattreatment is performed in the pressurized atmosphere of at least 1 MPaand less than 50 MPa.

In order to confirm the aforementioned effect, the inventors haveprepared oxide superconducting wires according to this embodiment underthe following conditions, for measuring critical current densities.

On the basis of the steps of manufacturing an oxide superconducting wireshown in FIG. 3, metal tubes were charged with raw material powder andsubjected to wire drawing. Then, tape-like superconducting wires wereobtained by performing primary rolling. The primary rolling wasperformed with two types of drafts of 82% and 87%. Further, rolls of 100mm in diameter and lubricating oil having kinetic viscosity of 10 mm²/swere employed for the primary rolling. Then, first heat treatment wasperformed on these wires. The first heat treatment was performed bysetting the partial oxygen pressure, the heat treatment temperature andthe heat treatment time to 0.008 MPa, 830° C. and 30 hours respectively.Then, secondary rolling was performed. The secondary rolling wasperformed with a draft of 5 to 30% and rolls of 300 mm in diameter, withno lubricating oil. Then, second heat treatment was performed. Thesecond heat treatment was performed by setting the total pressure, thepartial oxygen pressure, the heat treatment temperature and the heattreatment time to 25 MPa, 0.008 MPa, 820° C. and 50 hours respectively.After the second heat treatment, the critical current densities of theobtained oxide superconducting wires were measured.

Consequently, the oxide superconducting wire subjected to the primaryrolling with the draft of 87% exhibited a critical current density of 30kA/cm². On the other hand, the oxide superconducting wire subjected tothe primary rolling with the draft of 82% exhibited a critical currentdensity of 40 kA/cm². It is understood from the aforementioned resultsthat formation of gaps extending perpendicularly to the longitudinaldirection of the oxide superconducting wire can be suppressed before theheat treatment and the critical current density of the oxidesuperconducting wire can consequently be improved by setting the draftof the oxide superconducting wire to not more than 84% in the primaryrolling (step S5).

While the exemplary kinetic viscosity of the lubricating oil in rollingand the exemplary diameter of the rolls employed for rolling have beenshown in this embodiment, the present invention is not restricted tothese rolling conditions but the draft of the wire in the rolling stepmay simply be not more than 84%.

SIXTH EMBODIMENT

The inventors have further made a deep study, to find that formation ofgaps and blisters is further suppressed and reduction of the criticalcurrent density is effectively prevented also in the wire 1 having thepinholes 14 when a sixth technique described below is employed. Theyhave further found that ballooning can be further suppressed upontemperature increase without temperature control in an oxidesuperconducting wire manufactured with the sixth technique.

In the sixth technique, pressurization is started after the temperatureof the atmosphere exceeds 400° C., preferably 600° C., in temperatureincrease before heat treatment in either the first heat treatment (stepS4) or the second heat treatment (step S6) shown in FIG. 3. The pressingspeed is preferably set to at least 0.05 MPa/min., more preferably atleast 0.1 MPa/min.

FIG. 21 is a diagram showing exemplary relation between temperatures,total pressures and partial oxygen pressures in temperature increasebefore heat treatment and in the heat treatment and time in the sixthtechnique in a sixth embodiment of the present invention.

Referring to FIG. 21, the temperature of the atmosphere is slowlyincreased up to 820° C. The pressure of the atmosphere is slowlyincreased according to the state equation of gas when the temperature isless than 600° C. Pressurization is started after the temperature of theatmosphere reaches 600° C., and the pressurization is performed up toabout 25 MPa at a pressing speed of about 0.1 MPa/min. The partialoxygen pressure is kept in the rage of at least 0.003 MPa and less than0.02 MPa. The critical current density of an oxide superconducting wirecan be further improving by performing heat treatment under theseconditions. In order to confirm the effect of the aforementioned heattreatment method, the inventors have heat-treated oxide superconductingwires with various temperatures for starting pressurization as describedbelow, and measured sintering densities of the prepared oxidesuperconducting wires respectively.

FIG. 22 is a diagram showing the relation between pressing speeds andsintering densities with reference to various temperatures for startingpressurization.

Referring to FIG. 22, the sintering density of an oxide superconductorfilament (oxide superconductor) is about 93% to 96% with a pressingspeed of at least 0.05 MPa in a case of starting pressurization when thetemperature of the atmosphere is 30° C. In a case of startingpressurization when the temperature of the atmosphere reaches 400° C.,on the other hand, the sintering density of the oxide superconductorfilament is at least 95% with a pressuring speed of at least 0.05MPa/min. When pressurization is started after the temperature of theatmosphere reaches 600° C., further, the sintering density of the oxidesuperconductor filament is at least about 97% with a pressuring speed ofat least 0.05 MPa/min., and the sintering density of the oxidesuperconductor filament is at least about 98% with a pressing speed ofat least 0.1 MPa/min. In both cases of starting pressurization after thetemperature of the atmosphere reaches 400° C. and startingpressurization after the temperature reaches about 600° C., in addition,the sintering density of the oxide superconductor filament is at leastabout 99% with a pressing speed of at least 0.15 MPa/min.

The sintering density is conceivably improved with the pressing speed ofat least 0.05 MPa since the speed of the pressurizing gas infiltratinginto the wire through pinholes is less than about 0.05 MPa/min. and thewire is pressurized at a speed higher than this infiltration speed sothat the pressure in the atmosphere can regularly be kept higher thanthe internal pressure of the wire. According to the results shown inFIG. 12, the sintering density of the oxide superconductor filament isimproved when pressurization is started after the temperature of theatmosphere exceeds 400° C., preferably 600° C. Further, it is understoodthat the sintering density of the oxide superconductor filament isfurther improved when the pressing speed is preferably set to at least0.05 MPa/min., more preferably to at least 0.1 MPa/min. This isconceivably for the following reason:

FIG. 23 is a diagram showing temperature dependency of 0.2% yieldstrength of silver.

Referring to FIG. 23, the 0.2% yield strength is about 370 MPa when theatmosphere is at the room temperature, and reduced following temperaturerise in the atmosphere. More specifically, the 0.2% yield strength isreduced to about 50 MPa when the temperature of the atmosphere reaches400° C., and the 0.2% yield strength is reduced to about 25 MPa when thetemperature of the atmosphere reaches 600° C. Thus, the 0.2% yieldstrength of silver is reduced to a degree substantially identical to thetotal pressure (at least 1 MPa and less than 50 MPa) of theaforementioned pressurized atmosphere when the temperature of theatmosphere is 400° C. When the temperature of the atmosphere is 600° C.,the 0.2% yield strength of silver is reduced to about half the totalpressure (at least 1 MPa and less than 50 MPa) of the aforementionedpressurized atmosphere. According to the aforementioned technique, itfollows that a pressure is applied to the wire in a state where thestrength of a sheath portion is reduced. Therefore, the sheath portionis easily compressed by compressive force resulting from pressurizationthrough an effect similar to that of hot working. Consequently, the wireis compressed before pressurizing gas infiltrates into the wire throughpinholes, whereby formation of gaps and blisters can be sufficientlysuppressed by pressurization for improving the sintering density of theoxide superconductor filament. The values of the 0.2% yield strengthshown in FIG. 23 were obtained by performing a tensile test defined inJIS (Japan Industrial Standard) on a pure silver wire of 1.5 mm indiameter.

The sintering density of the oxide superconductor filament in FIG. 22 iscalculated by the following method: First, an oxide superconducting wireof 5 g (=M_(t) (g)) is dissevered. Then, the dissevered oxidesuperconducting wire is dipped in alcohol, for measuring the weight (W(g)) of the wire in alcohol and calculating buoyancy acting on the oxidesuperconducting wire. The volume (V_(t) (cm³)) of the oxidesuperconducting wire is calculated with known alcohol density (p=0.789(g/cm³)). More specifically, V, is calculated through the followingformulas (1) and (2), assuming that F_(t) represents the buoyancy:F _(t) =M _(t) −W  (1)V _(t) =F _(t)/ρ  (2)

Then, the oxide superconducting wire is dissolved in nitric acid so thatthe solution is subjected to ICP (inductive coupled plasma) emissionspectroscopy, thereby determining silver and calculating the ratio (Y)of silver in the weight of the oxide superconducting wire. The weight(M_(t) (g)) of an oxide superconductor filament portion and the weight(M_(s) (g)) of a sheath portion are calculated from the weight of theoxide superconducting wire through the following formulas (3) and (4):M _(s) =M _(t) ×Y  (3)M _(f) =M _(t) −M _(s)  (4)

Then, the volume (V_(s) (cm³)) of the sheath portion is calculated fromthe known silver density (10.5 (g/cm³)), and the volume (V_(f) (cm³)) ofthe oxide superconductor filament is calculated from the volume of thesheath portion. Further, the density ρ_(f) of the oxide superconductorfilament is calculated from the volume of the oxide superconductorfilament. More specifically, ρ_(f) is calculated through the followingformulas (5) to (7):V _(s) =M _(s)/10.5  (5)V _(f) =V _(t) −V _(s)  (6)ρ_(f) =M _(f) /V _(f)  (7)

On the other hand a value 6.35 g/cm³ is employed as the theoreticaldensity of the oxide superconductor filament. This value is calculatedby the following method: The atomic ratios of a Bi2223 phase in theoxide superconductor filament are calculated by ICP emissionspectroscopy and EDX (energy dispersive X-ray spectroscopy). The latticeconstant of the Bi2223 phase is obtained by X-ray diffraction, forcalculating the values of a- and c-axes. The theoretical density iscalculated from these values.

The sintering density of the oxide superconductor filament is calculatedfrom the ratio between the density of the oxide superconductor filamentand the theoretical density of the oxide superconductor filamentobtained by the aforementioned method. More specifically, the sinteringdensity is calculated through the following formula (8):sintering density (%)=(ρ_(f)/6.35)×100  (8)

FIG. 24 is a diagram showing the relation between sintering densities ofoxide superconductor filaments and critical current values of oxidesuperconducting wires.

Referring to FIG. 24, the critical current values of oxidesuperconducting wires having sintering densities of not more than about95% are less than 80 A, while the critical current values of oxidesuperconducting wires having sintering densities of at least about 95%are mainly in a range exceeding 80 A. The critical density value isobtained by multiplying the critical current density by the sectionalarea of the oxide superconductor filament, and hence the criticalcurrent density is proportionate to the critical current value.Therefore, the critical current density is improved in an oxidesuperconducting wire having a high sintering density. This isconceivably because a large quantity of current flows through thesuperconductor filament since the number of gaps between crystals of thesuperconductor filament is small in the oxide superconducting wirehaving a high sintering density.

From the aforementioned results shown in FIGS. 22 and 24, it isunderstood that the sintering density of the oxide superconductorfilament is improved to improve the critical current density of theoxide superconducting wire when pressurization is started after thetemperature of the atmosphere exceeds 400° C., more preferably 600° C.,preferably at a speed of at least 0.05 MPa/min., more preferably atleast 0.1 MPa/min.

An oxide superconducting wire having a sintering density of at least95%, preferably at least 99%, is obtained by the aforementionedmanufacturing method. The oxide superconducting wire obtained by theaforementioned manufacturing method is so applied to a superconductingdevice such as a superconducting cable that a liquid refrigerant furtherhardly infiltrates into gaps of an oxide superconductor. Also when thesuperconducting device is heated from a state dipped in the liquidrefrigerant to the room temperature without temperature control,therefore, the liquid refrigerant is hardly vaporized. Consequently, theinternal pressure of the oxide superconducting wire is so hardlyincreased that the superconducting device such as a superconductingcable can be further inhibited from ballooning.

In order to confirm the aforementioned effect, the inventors have madethe following experiment:

Two types of oxide superconducting wires were prepared through themanufacturing method shown in FIG. 3. The first oxide superconductingwire was heat-treated at a temperature of 820° C. for 50 hours with apressure of 30 MPa and a partial oxygen pressure of 0.008 MPa in secondheat treatment (step S6). In temperature increase before the second heattreatment (step S6), pressurization was started after the temperature inthe atmosphere reached 600° C., with control substantially similar tothe control of the total pressure, the partial oxygen pressure and thetemperature shown in FIG. 21. The second oxide superconducting wire washeat-treated with the atmospheric pressure in both of first heattreatment (step S4) and second heat treatment (step S6). Superconductingcables 30 similar to that shown in FIG. 1A were prepared from the twotypes of oxide superconducting wires obtained in the aforementionedmanner respectively. The respective superconducting cables 30 weredipped in liquid nitrogen for 24 hours, and heated to the roomtemperature without controlling the rate of temperature increase.Thereafter presence/absence of ballooning was investigated.Consequently, the superconducting cable 30 formed by the oxidesuperconducting wire heat-treated with the atmospheric pressure wasballooned. On the other hand, the superconducting cable 30 employing theoxide superconducting wire subjected to the control of the totalpressure, the partial oxygen pressure and the temperature similar tothat shown in FIG. 21 exhibited absolutely no ballooning. Thus, it isunderstood that a superconducting cable having an oxide superconductingwire manufactured by the aforementioned method can be inhibited fromballooning.

In the method of manufacturing an oxide superconducting wire accordingto this embodiment, a pressure is applied to the wire in such a statethat the 0.2% yield strength of the sheath portion is reduced to adegree substantially identical to the total pressure of a pressurizedatmosphere in heat treatment. Thus, the sheath portion is easilycompressible by compressive force resulting from pressurization due toan effect similar to that of hot working. Therefore, the wire iscompressed before pressurizing gas infiltrates into the wire throughpinholes, whereby formation of gaps and blisters can be sufficientlysuppressed by pressurization. Consequently, the sintering density of theoxide superconductor filament can be improved for improving the criticalcurrent density of the oxide superconducting wire.

Preferably in the aforementioned manufacturing, pressurization isstarted after the temperature of the atmosphere exceeds 600° C. intemperature increase before heat treatment in the heat treatment step.

Thus, the pressure is applied to the wire in a state where the 0.2%yield strength of the sheath portion is reduced to about half the totalpressure of the pressurized atmosphere in the heat treatment. Therefore,the sheath portion is further easily compressible by the compressiveforce resulting from pressurization. Consequently, the sintering densityof the oxide superconducting wire filament can be further improved forfurther improving the critical current density of the oxidesuperconducting wire.

Preferably in the aforementioned manufacturing method, the pressingspeed is at least 0.05 MPa/min., more preferably at least 0.1 MPa/min.

Thus, the sintering density of the oxide superconductor filament can befurther improved for further improving the critical current density ofthe oxide superconducting wire.

Preferably in the aforementioned manufacturing method, the heattreatment step is carried out in an oxygen atmosphere, and the partialoxygen pressure is at least 0.03 MPa and not more than 0.02 MPa.

Thus, a stable oxide superconducting phase is so formed that thecritical current density can be improved. A hetero phase is formed ifthe partial oxygen pressure exceeds 0.02 MPa, while the oxidesuperconducting wire is hard to form and the critical current density isreduced if the partial oxygen pressure is less than 0.003 MPa.

The method (method of manufacturing an oxide superconducting wire) ofimproving the critical current density by performing a prescribed heattreatment method in at least either the first heat treatment (step S4)or the second heat treatment (step S6) shown in FIG. 3 has been shown inthis embodiment. However, the present invention is also applicable as aheat treatment step performed on the manufactured oxide superconductingwire (i.e., the oxide superconducting wire after completion of the stepsS1 to S6 in FIG. 3), i.e., as a method of reforming an oxidesuperconducting wire, in addition to this case. Also when the heattreatment according to the present invention is employed as a method ofreforming an oxide superconducting wire, the sintering density of anoxide superconductor can be improved for improving the critical currentdensity of the oxide superconducting wire.

The case of heat-treating the oxide superconducting wire having thesheath portion of silver in the pressurized atmosphere having the totalpressure of at least 1 MPa and less than 50 MPa in the heat treatmentand starting pressurization after the temperature of the atmosphereexceeds 400° C. in the temperature increase before the heat treatment inthe heat treatment step has been shown in this embodiment. However, thepresent invention is not restricted to this case but is applicable to ageneral oxide superconducting wire having a configuration obtained bycovering an oxide superconductor with a metal. In this case, heattreatment is performed in a pressurized atmosphere having a totalpressure of at least 1 MPa and less than 50 MPa in the heat treatment,and pressurization is started from a temperature at which at least the0.2% yield strength of the metal is smaller than the total pressure (atleast 1 MPa and less than 50 MPa) in the heat treatment. Thus, apressure is applied to the wire in a state where the 0.2% yield strengthof the metal is smaller than the total pressure of the pressurizedatmosphere in the heat treatment, whereby a portion of the metal iseasily compressible by compressive force resulting from pressurization.Therefore, the sintering density of the oxide superconducting wire canbe improved for improving the critical current density of the oxidesuperconducting wire for a reason similar to that in the aforementionedoxide superconducting wire having the sheath portion of silver.

SEVENTH EMBODIMENT

A bismuth (Bi)-based oxide superconducting wire is generally known asone of oxide superconducting wires. This Bi-based oxide superconductingwire can be used at the liquid nitrogen temperature, and can attain arelatively high critical current density. Further, this Bi-based oxidesuperconducting wire, which can be relatively easily elongated, isexpected for application to a superconducting cable or magnet. However,there has been such a problem that a conventional Bi-based oxidesuperconducting wire is unsuitable for application requiring highperformance under a low temperature due to a low critical currentdensity (J_(c)) under a low temperature of about 20 K.

In this relation, the inventors have found that the critical currentdensity of a Bi-based oxide superconducting wire at a low temperature ofabout 20 K can be improved by combining the following technique with theaforementioned techniques. This technique is now described.

In this technique, a wire is annealed in an atmosphere containing oxygenat a temperature of at least 100° C. and not more than 600° C. in atleast either the first heat treatment (step S4) or the second heattreatment (step S6) shown in FIG. 3.

FIG. 25 is a diagram showing exemplary relation between temperatures,total pressures and partial oxygen pressures in a case of performingannealing after heat treatment and time in a seventh embodiment of thepresent invention.

Referring to FIG. 25, an oxide superconducting wire is held for aconstant time in a state where the temperature of the atmosphere is 820°C. and the pressure is 25 MPa, and the temperature of the atmosphere isthereafter reduced. At this time, the total pressure of the atmosphereis also slowly reduced. The wire is held at a constant temperature whenthe temperature and the pressure of the atmosphere reach about 300° C.and about 16 MPa respectively, and annealed for about 30 hours. Whilethe wire is held at the constant temperature, the total pressure isfurther continuously reduced slowly. The temperature of the atmosphereis reduced again after completion of the annealing. The partial oxygenpressure is about 0.008 MPa during the heat treatment, and increased toabout 0.024 MPa during the annealing. The partial oxygen pressure isreduced along with the total pressure after the annealing.

In order to confirm the effect of the aforementioned annealing, theinventors have made the following experiment:

They have investigated how much critical current values at 20 K areimproved in a case of performing annealing and a case of performing noannealing in heat treatment steps. The annealing was performed withvarious annealing times and various partial oxygen pressures. Table 3shows average values of rates of increase of critical current values at22 K with respect to critical current values at 77 K after heattreatment steps as to respective samples. The critical current valueswere measured in a magnetic field of 3 T. TABLE 3 Partial Average:Sample Oxygen Ic(20 K)/ No. Temperature Time Pressure Ic(77 K) 1unannealed 1.6 2 unannealed 1.7 3 unannealed 1.5 4 annealed 300° C. 30 h24 kPa 2.1 5 annealed 300° C. 30 h 12 kPa 1.9 6 annealed 300° C. 40 h 20kPa 2

Referring to Table 3, the average values of the rates of increase of thecritical current values at 22 K in the case of performing no annealingare 1.6, 1.7 and 1.5 respectively. On the other hand, the average valuesof the rates of increase of the critical currents at 22 K in the case ofperforming annealing are 2.1, 1.9 and 2 respectively. Therefore, it isunderstood that the critical current value at 20 K can be more improvedin the case of performing annealing, as compared with the case ofperforming no annealing. No change of I_(c) at 77 K was recognized.

In order to confirm the effect of annealing the wire in the atmospherecontaining oxygen at the temperature of at least 100° C. and not morethan 600° C., the inventors have made the following experiment:

First, tape-like Bi-based oxide superconducting wires each having amultifilamentary structure provided with 61 filaments with externalsizes of 4.2 mm in width and 0.24 mm in thickness and a silver ratio of1.5 were prepared. Further, heat treatment was performed on these oxidesuperconducting wires, and annealing was performed in this heattreatment. The annealing was performed in an oxygen jet for an annealingtime of 20 hours at various annealing temperatures shown in Table 4.Further, oxide superconductors were prepared with various quantities ofBi2212 phases ((BiPb)₂Sr₂Ca₁Cu₂O_(8+z) superconducting phases). Table 4also shows critical current values I_(c) of the respective samples at 77K and 20 K before and after annealing respectively.

The used wires were selected from the same lot, and it is assumed thatsuperconducting portions of all wires have the same sectional area.Thus, the magnitudes of the critical current values I_(c) in Table 4 areproportionate to the critical current densities J_(c)(J_(c)=I_(c)/sectional area of superconducting portion). TABLE 4Quantity of Annealing Sample Bi2212 Phase Before Annealing BeforeAnnealing Temperature After Annealing After Annealing No. (%) 77 K Ic(A)20 K Ic(A)(1) (° C.) 77 K Ic(A) 20 K Ic(A)(2) (2)/(1) 7 9 95 500 none —— — 8 9 95 500 100 95 515 1.03 9 9 95 500 200 95 535 1.07 10 9 95 500300 94 545 1.09 11 9 95 500 400 92 550 1.1 12 9 95 500 500 90 575 1.1513 9 95 500 600 89 550 1.1 14 9 95 500 700 70 480 0.96 15 9 95 500 80060 345 0.69 16 2 100 527 500 99 528 1.0 17 5 97 511 500 96 543 1.06 18 995 500 500 90 555 1.11 19 13 92 485 500 88 540 1.11 20 19 90 474 500 82530 1.12 21 25 83 437 500 75 500 1.14 22 50 60 316 500 50 410 1.3

It is understood from the results in Table 4 that the critical currentvalue I_(c) (critical current density J_(c)) at a low temperature (20 K)is more improved as compared with that before annealing when theannealing is performed in an oxygen atmosphere at a temperature of atleast 100° C. and not more than 600° C. Particularly when the annealingtemperature is at least 300° C. and not more than 600° C. and thequantity of the Bi2212 phase in the oxide superconductor is at least 5mol % and not more than 20 mol %, the critical current value I_(c) afterthe annealing is at least 530 A, and it is understood that the absolutevalue of the critical current value I_(c) (critical current densityJ_(c)) is increased.

They have also investigated critical current values I_(c) of oxidesuperconducting wires at respective temperatures (K) before annealingand after annealing at a temperature of 500° C. FIG. 26 shows theresults. It is understood from the results show in FIG. 26 that thecritical current values I_(c) of the annealed samples are higher thanthose of the samples not yet subjected to annealing from a temperatureof not more than about 20 K.

In the method of manufacturing an oxide superconducting wire accordingto this embodiment, the oxide superconducting wire includes a Bi2223phase, and the oxide superconducting wire is annealed in an atmospherecontaining oxygen at a temperature of at least 100° C. and not more than600° C. Thus, the critical current density of the oxide superconductingwire is improved at a low temperature of about 20 K.

The method of improving the critical current density by performing aprescribed heat treatment method in at least either the first heattreatment (step S4) or the second heat treatment (step S6) shown in FIG.3 has been shown in this embodiment. However, the present invention isalso applicable as a heat treatment step performed on the manufacturedoxide superconducting wire (i.e., the oxide superconducting wire aftercompletion of the steps S1 to S6 in FIG. 3), i.e., as a method ofreforming an oxide superconducting wire, in addition to this case. Alsowhen the heat treatment according to the present invention is employedas a method of reforming an oxide superconducting wire, the criticalcurrent density of the oxide superconducting wire can be improved at alow temperature of about 20 K.

The present invention is applicable to a superconducting device such asa superconducting transformer, a superconducting current limiter or amagnetic field generator employing a superconducting magnet composed ofan oxide superconducting wire or a superconducting cable, asuperconducting bus bar or a superconducting coil employing an oxidesuperconducting wire, and particularly applicable to a superconductingdevice in which an oxide superconducting wire is used in a state dippedin a refrigerant. Further, the present invention can effectivelysuppress ballooning particularly when applied to a superconducting cableamong superconducting devices.

The embodiments described in the above must be considered illustrativeand not restrictive in all points. The range of the present invention isshown not by the aforementioned embodiments but by the scope of claimfor patent, and intended to include all corrections and modificationswithin the meaning and range equivalent to those of the scope of claimfor patent.

1. A superconducting device having an oxide superconducting wire with anoxide superconductor exhibiting sintering density of at least 93%,wherein said oxide superconductor is a Bi—Pb—Sr—Ca—Cu—O-based oxidesuperconductor containing bismuth lead, strontium, calcium and copperand including a Bi2223 phase having atomic ratios of (bismuth andlead):strontium:calcium:copper expressed as 2:2:2:3 in approximation. 2.The superconducting device according to claim 1, having said oxidesuperconducting wire with said oxide superconductor exhibiting saidsintering density of at least 95%.
 3. The superconducting deviceaccording to claim 2, having said oxide superconducting wire with saidoxide superconductor exhibiting said sintering density of at least 99%.4. A superconducting cable having an oxide superconducting wire with anoxide superconductor exhibiting sintering density of at least 93%wherein said oxide superconductor is a Bi—Pb—Sr—Ca—Cu—O-based oxidesuperconductor containing bismuth, lead, strontium, calcium and copperand including a Bi2223 phase having atomic ratios of (bismuth andlead):strontium:calcium:copper expressed as 2:2:2:3 in approximation. 5.The superconducting cable according to claim 4, having said oxidesuperconducting wire with said oxide superconductor exhibiting saidsintering density of at least 95%.
 6. The superconducting cableaccording to claim 5, having said oxide superconducting wire with saidoxide superconductor exhibiting said sintering density of at least 99%.